Reassuring allies in times of crisis: assessing the effectiveness of tools for alliance reassurance in crises

Abstract

What military and diplomatic instruments are effective for the United States to reassure its allies that their security will be protected as promised? Despite its importance for alliance management, our understanding of alliance reassurance remains incomplete. Specifically, existing research rarely uses direct measures of reassurance outcomes, and pays limited attention to exploring the effectiveness of different toolkits for alliance reassurance. Using a survey experiment with South Korean citizens, this paper examines the effectiveness of a range of military and diplomatic toolkits for reassuring US allies during foreign policy crises. The results demonstrate that conventional and nuclear signals, as well as diplomatic statements of support, significantly assuage the security concerns of South Korean citizens by increasing the perceived safety of South Korea from North Korean military aggression. However, Washington’s diplomatic proposals toward North Korea do not produce similar effects. These findings have implications for debates on interstate signaling and alliance management.

Introduction

What military and diplomatic policy instruments are effective for the United States to reassure its allies that it will not allow their security to be harmed? Since the end of the Second World War, military alliances have served key strategic interests of the United States, such as forward defense (e.g. Rapp-Hooper, 2020), extended deterrence (e.g. Brooks et al., 2012: 11), policy leverage in other issue areas (e.g. Mastanduno, 2009), and allied nuclear nonproliferation (e.g. Bleek and Lorber, 2014). Given the significance of the allies’ role in US grand strategy, convincing them that relying on US security commitment is the best way to protect their security is a critical aspect of managing the US-led alliance system.

Achieving this goal of alliance reassurance, defined as convincing an ally that its security will be protected as promised, is a daunting task, however. Weak US support could make allies worried that the United States does not have the willingness to honor its security commitments. As British defense minister Denis Healey noted, “It takes only five per cent credibility of American retaliation to deter the Russians, but ninety-five per cent credibility to reassure the Europeans” (Healey, 1989: 243). On the other hand, confrontational actions that could provoke adversaries make allies concerned over the possibility that they might be dragged into unwanted conflicts by Washington’s aggressive moves. Convincing allies that US security guarantees can effectively address these risks of abandonment and entrapment (Snyder, 1997) is a key to successful alliance reassurance (Sukin and Lanoszka, 2024).

Understanding the utility of different toolkits for alliance reassurance has significant implications for US grand strategy and foreign policy. Unsuccessful reassurance efforts could lead a US ally facing security threats to seek alternative policy options, such as realignment, neutrality, or an independent nuclear deterrent. Relatedly, the success and failure of alliance reassurance may have broader implications beyond the specific bilateral relationship between the United States and a given ally. Depending on circumstances, other allies may attempt to infer information about US capability, resolve, and foreign policy interests from US behavior toward the adversary and the ally (e.g. Kim and Simón, 2021). Thus, whether US reassurance efforts succeed or fail could also create far-reaching consequences for these allies’ inference about Washington’s reliability in future crises.

Despite the importance of alliance reassurance, our understanding of what policy options are effective for reassuring allies remains limited. To date, scholarly attention has mostly centered on the success and failure of extended deterrence without directly exploring what explains alliance reassurance outcomes (e.g. Fuhrmann and Sechser, 2014; Huth, 1988; Huth and Russett, 1984), although deterring adversaries and reassuring allies are equally important for alliance management (e.g. Snyder, 1997: 329–331). Moreover, in the literature on nuclear proliferation, alliance reassurance has been typically theorized as a causal mechanism through which security guarantees reduce the nuclear interest of allies (Bleek and Lorber, 2014; Ko, 2019; Lanoszka, 2018; Reiter, 2014). As Blankenship and Lin-Greenberg (2022: 94) aptly note, however, this literature rarely measures reassurance outcomes directly, since it considers achieving desired outcomes (e.g. nonproliferation) an indirect indicator of successful reassurance. Finally, although it provides useful theoretical insights into the determinants of reassurance success, the literature on interstate signaling tends to place its empirical emphasis on the sender-adversary relationship (e.g. Kertzer et al., 2020, 2021; Yarhi-Milo et al., 2018). Consequently, whether the key results from this literature apply to the sender-ally relationship remains underexplored.

This paper examines the effect of US military and diplomatic actions on alliance reassurance when allies are embroiled in foreign policy crises. Existing studies on alliance reassurance tend to focus on alliance reassurance in peacetime (e.g. Blankenship and Lin-Greenberg, 2022; Ko, 2019; Reiter, 2014). Yet studying how to reassure an ally facing an acute foreign policy crisis offers important contributions. For instance, it is open to debate whether tools for peacetime reassurance are as effective for reassuring allies embroiled in crises as they are in peacetime because the outbreak of a crisis indicates the failure of peacetime deterrence, which could undermine the perceived ability of the United States to deter the escalation of the crisis. The imminent nature of the adversary’s threats during crises, as opposed to the latent prospect of aggression by the adversary during peacetime, might also increase the threshold for a policy instrument to effectively address the ally’s security concerns. An empirical investigation of the effectiveness of various policy tools for reassurance during crises can provide important insights into the utility of those toolkits in different contexts.

In this sense, Sukin and Lanoszka (2024) make a welcome contribution providing important findings on the policy preferences of citizens of US allies in Central-Eastern Europe regarding Washington’s reassurance toolkits under crisis conditions. They find that those citizens prefer less risky instruments, such as sanctions and public reiteration of guarantees, to policies that might increase entrapment risks, such as military demonstrations. Not only do these findings reveal important insights into allies’ preference ranking regarding Washington’s reassurance instruments, but they also show that conceptually, alliance reassurance is closely related to both the fear of abandonment and entrapment, a key insight from Snyder (1997) that is sometimes overlooked by past studies on alliance reassurance.

Building on previous works, this paper examines the effects of four policy instruments for alliance reassurance by using a scenario-based survey experiment with South Korean respondents: conventional military signals, nuclear signals, statements of support, and diplomatic overtures toward adversaries. Although I do not argue that those tools constitute a comprehensive list of tools for alliance reassurance, they have been used by US policymakers in their efforts to reassure allies. They are also closely related to key arguments from the scholarly literature on interstate signaling, which provides a sound starting point for understanding the effects of tools for alliance reassurance.

This paper makes several contributions to the literature on alliance reassurance. First, it offers a clear theoretical framework that defines alliance reassurance and specifies allies’ primary concerns during crises. By doing so, it clarifies what reassurance signals are meant to achieve and accommodates different perspectives on what reassures allies. Second, it examines a range of toolkits for reassurance across different domains, such as military and diplomatic domains. Even in the military domain, where most studies focus on how conventional military forces contribute to reassurance (e.g. Blankenship and Lin-Greenberg, 2022; Sukin and Lanoszka, 2024), I explore the utility of nuclear weapons as a reassurance tool. The role of nuclear weapons in reassuring allies has been contested, and there has been an increasingly skeptical voice on the impact of nuclear reassurance (e.g. Mount, 2023; Sukin and Dalton, 2021). This paper contributes to this debate by laying out the theoretical logic of why reassurance signals with nuclear-capable forces could be useful for reassuring allies and providing micro-level evidence on whether nuclear signaling helps the United States reassure its allies. Third, this paper broadens the empirical scope of survey-based research on alliance reassurance, which mostly focuses on the European context (Blankenship and Lin-Greenberg, 2022; Sukin and Lanoszka, 2024), by studying a US ally in East Asia.

My analysis of data from a survey experiment based on a hypothetical crisis between the United States, South Korea, and North Korea finds that US conventional and nuclear signals, as well as its public statements of support, successfully convince South Korean respondents that South Korea would be safe from potential North Korean aggression. I also find that there are almost no meaningful differences in the relative utility of military signals and statements of support for reassuring South Korean citizens. However, Washington’s public overtures for diplomatic solutions fail to produce a comparable reassurance effect. Finally, the reassurance effects of military and diplomatic instruments vary across several individual-level characteristics, such as foreign policy orientation, the perceived foreign policy intentions of North Korea, support for a South Korean nuclear deterrent, and political viewpoints.

Although foreign policy decision-making is often considered an elite-driven phenomenon, survey experiments with ordinary citizens can produce valuable insights. First, mass public and foreign policy elites are two key audiences of US reassurance efforts directed at a given ally. Thus, examining what makes the ally’s public successfully reassured is a first step toward a fuller understanding of how the United States can mitigate its allies’ security concerns. Second, there is considerable evidence against the argument that elites’ decision-making is systematically different from the mass public’s decision-making (e.g. Kertzer, 2022; Kertzer et al., 2021). It suggests that both elites and ordinary citizens may similarly respond to external events. If this is the case, then studying ordinary citizens can still produce valuable insights into foreign policy decision-making, even if most foreign policy decisions are made by elites. Third, the public plays an important role in shaping a state’s foreign policy choices. There is a well-established argument about the public’s influence on democracies’ foreign policy (e.g. Tomz et al., 2020), and studies also demonstrate that citizens’ preferences could be influential in non-democratic states’ foreign policy (e.g. Weiss and Dafoe, 2019). As such, understanding how allies’ mass public responds to Washington’s reassurance efforts helps improve our knowledge of alliance reassurance.

The remainder of this paper proceeds as follows. First, I explain my definition of alliance reassurance and a theoretical framework of what drives the successful reassurance of allies during peacetime and crises. Second, I discuss military and diplomatic toolkits the United States can use to reassure allies embroiled in crises and develop hypotheses on the utility of each option. Third, I describe the key elements of my research design. Fourth, I show the results from my survey experiment. I conclude with a discussion of the scholarly and policy implications of my findings.

Alliance reassurance in crises: a theoretical framework

Definition

I begin by defining alliance reassurance. ¹ In this paper, alliance reassurance is defined as convincing an ally that its security will be protected from external threats as promised. This definition builds on existing works on alliance reassurance, which commonly highlights that making promises to protect security is the primary goal of acts of reassurance (e.g. Blankenship and Lin-Greenberg, 2022: 93; Knopf, 2012: 376, 378; Rapp-Hooper, 2020: 57). Depending on the relationship with the recipient, reassurance could be labeled as alliance reassurance, which is directed at members of a formal alliance, or adversary reassurance, which is directed at an adversary to address the adversary’s security concerns (Rapp-Hooper, 2015: 128).

For a policy of alliance reassurance to be effective, it should successfully address the sources of threats to allies’ security. What this means for the United States depends on whether alliance reassurance is attempted in peacetime or crisis. During peacetime, the United States primarily exercises extended general deterrence: to prevent its ally’s adversary from challenging the ally’s security interests. If this effort fails and the ally is involved in a military crisis, then the primary goal of the United States is extended immediate deterrence: to prevent the adversary from escalating the ongoing crisis and attacking the ally (Huth and Russett, 1984: 496–497). Therefore, peacetime alliance assurance primarily aims to convince the ally that the likelihood of a crisis initiated by an adversary would remain a remote possibility. On the other hand, intra-crisis alliance reassurance aims to alleviate the ally’s security concerns by convincing it that the crisis would not escalate to military conflict.

The dynamics of intra-crisis alliance reassurance may be similar to those of peacetime reassurance, which has been studied by recent works (e.g. Blankenship and Lin-Greenberg, 2022; Ko, 2019; Reiter, 2014). But there are theoretical and policy reasons why we still need to study intra-crisis alliance reassurance separately. It is underexplored, for example, whether actions that contribute to peacetime reassurance are also effective for intra-crisis reassurance. The fact that a US ally is entangled in a foreign policy crisis implies that previous US attempts to prevent the ally’s adversary from threatening its ally’s security already failed, which could make successful intra-crisis reassurance a challenging task. In addition, the imminent nature of security threats posed by the adversary during crises, in contrast with peacetime where the adversary’s threats largely remain latent, might also make intra-crisis reassurance harder to succeed. Finally, the policy implications of understanding the success and failure of reassuring allies in crises may be particularly crucial as the ramifications of reassurance failure could be long-lasting and dramatically affect allies’ calculations. As Snyder (1997: 180) notes, in the context of alliance management, “short-term tactics and outcomes are bound to affect the long-term relationship and may be chosen in part with the long term in mind.” The fact that the United States fails to fulfill its promise to protect the ally’s security at the very moment when the ally needs US support the most might strongly drive the ally’s pursuit of alternative strategies and significantly damage US reputation as a reliable security provider.

What reassures allies? Insights from theories of crisis bargaining

What explains the effectiveness of measures of alliance reassurance? Like deterrence, the success of alliance reassurance is in the eye of the beholder (Knopf, 2012: 383). Whether the United States can succeed in reassuring its ally depends on whether Washington takes actions that are believed by the ally to reduce external threats to its security. As discussed above, during crises, the possibility of crisis escalation and the outbreak of military conflict is the most important source of external threats to the ally. Therefore, the ally’s evaluation of US reassurance efforts is influenced by the ideas that audiences in the ally have about why this ongoing crisis occurs and what actions would be most effective for de-escalating the crisis.

One way of reassuring allies is to conduct actions that persuade allies to believe that the United States will keep its promises to intervene. If allies believe that taking a firm stance is needed to prevent the adversary from ratcheting up its challenge, then such measures will achieve reassurance success. However, if audiences in allies believe that the source of the ongoing crisis is the spiral of misperception among involved parties, then policies that address such a security dilemma, rather than attempts to increase commitment credibility, will lead to reassurance success. This is why defining alliance reassurance in terms of allies’ sense of security—rather than their perception of commitment credibility—is important. In principle, policies that are seen as increasing the odds of further escalation may not necessarily elevate audiences’ sense of security (Sukin and Lanoszka, 2024).

To summarize, successful intra-crisis alliance reassurance can be achieved by actions that are perceived by audiences to significantly decrease the chance of crisis escalation. By observing the actions that are consistent with their beliefs about how interstate crisis bargaining and management work, these audiences will be convinced that the crisis will not escalate to military conflict and their security will not be harmed.

Theories of crisis bargaining provide useful insights into specifics of the ideas that audiences in a US ally use when they assess the likely effects of US actions for alliance reassurance. To begin with, actions that aim to deter an adversary may also be useful for reassuring allies, as argued by recent work (e.g. Blankenship and Lin-Greenberg, 2022; Reiter, 2014). Here the underlying logic is that measures that increase the credibility, perceived by the adversary, of US commitments to come to the aid of its ally would help reassure the ally because the ally is mainly concerned about the United States not doing enough to deter the adversary. If audiences in the ally believe that crisis escalation is best avoided by US actions demonstrating its capability and firmness, actions for deterring the adversary would also work for alliance reassurance. As such, policies such as troop deployment and statements that promise to help the ally would alleviate the ally’s security concerns and successfully reassure it.

However, this “reassurance by deterrence” approach may not be the only approach that leads to effective alliance reassurance. For instance, the security dilemma theory argues that actions that aim to deter the adversary could inadvertently deepen the adversary’s security concerns, as it interprets these measures as evidence of hostile intentions (Jervis, 1976). Instead of being deterred, the adversary chooses to issue counter-threats or raises its military preparation, thus triggering a dangerous action-reaction process. If the ally believes that the best approach to avoiding crisis escalation is to address the adversary’s motivations for crisis-triggering actions through de-escalation measures, such as dialogue and negotiations, then signals for deterrence may increase the likelihood of crisis escalation, the very outcome that the ally would like to avoid. The ally may be concerned about the possibility that US actions to show its firmness may provoke the adversary, increase tensions, and eventually lead to crisis escalation and the occurrence of military conflict.

In fact, the idea that allies may not always support confrontational and hawkish actions by the United States is in line with a key argument from existing research on alliance politics: states often fear both the risk of being abandoned and being entrapped by their allies (Snyder, 1997). As a result, actions aimed at credibly demonstrating US firmness might inadvertently lead its allies to believe that these actions would drag them into an unwanted military confrontation. Recent research supports this argument by demonstrating that signals that are designed to increase commitment credibility could heighten allies’ desire for nuclear weapons because they are afraid of getting dragged into conflicts triggered by the United States (e.g. Sukin, 2020).

As a result, actions that are believed to solve the security dilemma, such as reassurance toward adversaries (Jervis, 1976; Lebow, 1985; Stein, 1991), may also be effective for alliance reassurance in times of crisis, in addition to measures for reassurance that are typically studied by existing research, such as military deployment, demonstration, and statements demonstrating firmness. Thus, it is important to examine the reassurance utility of policy options that are advocated by various schools of thought on interstate signaling and crisis bargaining.

Military and diplomatic tools for alliance reassurance

Based on the above discussion, I develop hypotheses on the effects of four policy options on reassuring allies during acute foreign policy crises: military signals employing conventional forces, military signals using nuclear-capable forces, public statements of support, and diplomatic overtures toward adversaries. Although there could be a myriad of short- and long-term policy options US policymakers could use to reassure their allies, I focus on short-term policy toolkits because they are particularly important in the context of intra-crisis alliance assurance.

As US alliance management strategies do not always solely rely upon military options, including both military and non-military tools for assurance would also allow us to explore the effectiveness of a broader range of policy options than often envisioned in the existing literature. ² These toolkits are also widely studied by research on crisis bargaining and alliance management, thereby promoting the application of well-established theoretical ideas to the analysis of specific policy issues.

While I examine the effect of each policy option on alliance assurance individually, it does not mean that those policy options are always used in isolation. The United States has often used more than one policy option to alleviate its allies’ security concerns. Moreover, it is also possible that each policy tool can interact with one another, producing even stronger (or weaker) reassurance effects (Goldgeier and Wojtowicz, 2022: 738–739). To understand the effectiveness of a variety of assurance policy packages or the potential interaction among different tools for reassurance, however, we first need a good knowledge of how each policy performs. Therefore, exploring the utility of individual measures is an important first step toward a fuller understanding of the success and failure of US reassurance efforts.

Military signals with conventional forces

First, the United States can use its conventional military forces as a tool for sending messages to its ally that Washington has the will and capability to deter an adversary from escalating a crisis. ³ It possesses a robust conventional capability for signaling toward its allies, including the geographically distant ones. Military assets such as carrier strike groups, long-range conventional bombers, and naval surface ships are often mobilized for forward deployment, military exercises, and military alerts to reassure US allies. For instance, when King Hussein of Jordan confronted security challenges from an attempt to overthrow his regime with alleged backing from Egypt and Syria in 1958, Washington sent two carrier battle groups, a battleship, along with several supporting elements to demonstrate its support for the Hussein regime (Siegel, 1991: 25). During the Trump administration, the United States sent its B-1B conventional long-range bombers to the Korean Peninsula to assure South Korea and Japan thirteen times in 2017 alone (Mount, 2017).

Conventional military signals can contribute to alliance reassurance in multiple ways. First, studies on interstate signaling argue that as a prominent example of costly signals, visible military actions credibly demonstrate Washington’s willingness to aid its ally or augment its capability for fighting. To begin with, by publicly implicating threats to use force, these actions could generate domestic audience costs (Fearon, 1994, 1997), making it harder for leaders not to implement their threats and thus increasing the likelihood of a US intervention. Actions such as forward military deployment are also financially costly, and those costs can convey a message that United States resolve to defend the ally is strong enough to readily accept such a financial burden (Fearon, 1997). These features of public military signals can assure the ally that the adversary would be deterred from escalating the crisis further.

Military signals, especially when involving major force movements, could also shift the local balance of power favorably (Slantchev, 2005, 2011) by increasing the sheer size or the readiness of available military forces. In this case, military signals augment the local combat capability of the allied force, thus decreasing the chance that the adversary would achieve its aims by force. ⁴ Therefore, observing US conventional signals in a crisis would make the ally’s citizens feel more secure as they would believe that the crisis would not escalate to war due to those signals.

• H1: US military signals using conventional forces will reassure the ally’s public.

Military signals with nuclear-capable forces

Nuclear forces are another military instrument the United States could use for reassurance goals. Capabilities such as nuclear-capable bombers and ballistic missile submarines have been used as a tool to reassure the allies of the United States when necessary. For instance, when North Korea conducted its third nuclear test and triggered a crisis for South Korea in 2013, the United States sent two B-2 nuclear-capable bombers to join the ongoing US–South Korea military exercise. The past White House Press Secretary Joshua Earnest stated that it intended to send the message that the United States “stand shoulder to shoulder with our allies in South Korea to ensure . . . that the interests of the United States and the allies of the United States remain protected” (White House Office of the Press Secretary, 2013).

The utility of nuclear weapons for alliance reassurance has been contested, and recent works raise an especially skeptical perspective on the impact of nuclear signals on reassuring allies (e.g. Mount, 2023; Sukin and Dalton, 2021). Yet, there are clear theoretical reasons to expect that military signals with nuclear forces could help the United States reassure its ally. Like conventional military signals, nuclear signals could incur financial and domestic political audience costs, thus creating pressures on the United States to live up to its commitments to allies. Nuclear signals could also help the United States reveal its resolve by increasing the risk of inadvertent nuclear escalation (e.g. Schelling, 1960: 187–203). The fact that those signals are sent could indicate that the United States is willing to bear even an exceptionally high level of risk to support its ally in contingencies.

Second, intra-crisis nuclear signals could also improve the chance of a successful nuclear strike by increasing the readiness of the mobilized forces and the size of the nuclear assets that could be quickly used in contingencies. For example, forward-deployed nuclear forces could contribute more to desirable battlefield outcomes (in case of tactical nuclear use) or prompt nuclear punishment than the forces deployed in distant regions by reducing the opponent’s response time. These expectations could have significant implications; if the adversary’s nuclear forces are small and already vulnerable to US counterforce operations, then the fear of losing tools for nuclear retaliation and coercion might discourage the adversary from engaging in counter-escalation. The prospect of a quick nuclear retaliation might also prevent the adversary from attempting to achieve a military fait accompli. If the citizens of the ally follow those logics, then they would consider US nuclear signals as contributing to the protection of their security by preventing further escalation.

• H2: US military signals using nuclear forces will reassure the ally’s public.

Public statements of support for US allies

Despite the conventional wisdom that actions speak louder than words, US policymakers often use public statements to express their intent to support US allies and convince them that they can count on Washington for the protection of their security. In 2019, for example, then Secretary of State Mike Pompeo stated during his meeting with the Philippines’ Foreign Secretary Teddy Locsin that “As the South China Sea is part of the Pacific, any armed attack on Philippine forces, aircraft or public vessels in the South China Sea will trigger mutual defense obligations under Article 4 of our mutual defense treaty” (Panda, 2019). In May 2022, after North Korea conducted its 14th missile test of the year, US President Joe Biden declared that the United States is “committed to strengthening our close engagement and work together to take on challenges of regional security, including addressing the threat posed by the Democratic People’s Republic of Korea by further strengthening our deterrence posture,” in a joint press conference with South Korean President Yoon Suk Yeol (White House Office of the Press Secretary, 2022).

According to theories of costly signals, public statements of support could be effective for alliance reassurance because they can bolster the credibility of US security commitments to its ally. Ordinary citizens in the ally would view US statements of support as effectively deterring the adversary’s further escalation because they believe key tenets of theories of costly signals. It has been argued that public promises credibly demonstrate the sender’s resolve by incurring domestic political punishment and reputational costs in case of failure to fulfill those promises (Fearon, 1994; McManus, 2017; Sartori, 2007). When observing US statements of support, therefore, the mass public would believe that the security of their country will be protected by those statements because the adversary would update the costs of military aggression upward and restrain itself from engaging in further escalation.

• H3: US statements of support for allies will reassure the ally’s public.

Diplomatic overtures

The three policy toolkits discussed above commonly aim to reassure a US ally by increasing the credibility of existing US security commitments. On the other hand, the fourth policy option, diplomatic overtures toward an adversary, intends to reassure the ally by alleviating the adversary’s anxiety and thus dissuading the adversary from escalating the crisis with the use of force. In doing so, it seeks to persuade the adversary that the United States does not have hostile intentions and is willing to start negotiated bargains to address the ongoing crisis peacefully without imposing coercive pressures. The ultimate goal of diplomatic overtures, like military signals and public statements of support, is to convince allies that the United States is a reliable security provider, capable of preventing the further escalation of the ongoing crisis. The process through which diplomatic proposals aim to achieve that goal, however, is different; while military signals and statements of support attempt to strengthen intra-crisis deterrence, diplomatic overtures attempt to reduce the adversary’s motivation for crisis-triggering challenges and thus reduce tensions. ⁵

The role of diplomatic overtures has received less attention from existing research on alliance reassurance, which tends to explore the role of military measures in reassuring allies (e.g. Blankenship and Lin-Greenberg, 2022). It is important to note, however, that Washington’s allies do not always believe that coercive options are most effective for reducing the severity of threats. Instead, they often emphasize the role of diplomatic negotiations in addressing the challenges posed by adversaries. As Brad Roberts (2016: 215–216) notes, US allies are

equally committed to the proposition that the United States should make every reasonable effort to use political and economic tools to try to reduce nuclear dangers—especially when threat reduction may pay local dividends in their regions. . . . Allies also seek assurance that U.S. and allied security strategies will be effective in convincing regional challengers to abandon confrontation and seek cooperation.

In other words, if reassuring allies embroiled in crises requires the United States to reduce the chance of crisis escalation, then diplomatic overtures toward adversaries could be another important tool for alliance reassurance by “lessening the adversary’s security concerns, reducing its incentive to attack out of fear” (Kydd and McManus, 2017: 326).

Overtures for negotiated settlements may be less frequently used than coercive signals, but their role in alliance reassurance is not negligible. In the late 1970s, for example, when Soviet deployment of SS-20 intermediate-range nuclear missiles increased security concerns of the North Atlantic Treaty Organization (NATO) member states, they asked the United States to pursue diplomatic negotiations to limit theater nuclear forces (Anderson and Nelson, 2019: 94). As another example, South Korea also encouraged the United States to use diplomatic options when facing the security threats posed by Pyongyang’s expansion of its nuclear capabilities with several rounds of ballistic missile tests. In the US–South Korea joint statement in 2021, it is stated that

President Moon welcomes the conclusion of the United States’ DPRK policy review, which takes a calibrated and practical approach that is open to and will explore diplomacy with the DPRK to make tangible progress that increases the security of the United States and the Republic of Korea. (White House Office of the Press Secretary, 2021)

Audiences in a US ally would be reassured by US diplomatic overtures if they view de-escalation through diplomacy as the best approach to avoiding crisis escalation. If they see the ongoing crisis as caused by the adversary’s insecurity and a resultant action–reaction spiral, then they would believe that US proposals for diplomacy, acting as “adversary reassurance,” could convince the adversary that diplomatic negotiations would be a viable alternative by conveying US willingness to negotiate important issues (Stein, 1991: 433). It could also mitigate the adversary’s fear of US military attack, thus reducing its incentive to act first. Huth (1988: 431) also notes that diplomatic proposals could act as a “policy of reciprocity,” offering “a way for the potential attacker to back down in the confrontation with minimum damage to its bargaining reputation.” As a result, diplomatic overtures toward the adversary contribute to alliance reassurance by reducing its motivations to use force during the crisis.

• H4: US diplomatic overtures toward the adversary will reassure the ally’s public.

Research design

To test these hypotheses on alliance reassurance, I use a pre-registered survey experiment with South Korean respondents. ⁶ One of the key benefits of using experimental approaches is that they help researchers address a key inferential challenge to studies on alliance reassurance using observational data: measures for reassurance are likely to be chosen strategically. If the United States uses certain policy tools for reassuring allies based on its belief about the expected effect of those tools, which are shaped by other observable factors, then the observed relationship between reassurance toolkits and their effectiveness may be confounded. By randomly assigning respondents to each treatment arm, an experiment embedded in survey research could help researchers address this challenge.

Comparing the control group to each treatment group allows us to infer the effect of each treatment condition. In addition, by comparing one treatment group to another treatment group, we can also investigate whether the effectiveness of one policy instrument for alliance reassurance is greater than another policy option. Given that different policies entail different costs and risks, answers to this question will advance our knowledge of alliance reassurance. For instance, nuclear signals may contribute to alliance reassurance, but in doing so, it could also raise the risk of nuclear escalation and the salience of nuclear weapons in general, which could create adverse consequences for nuclear proliferation (e.g. Sukin, 2020; Sukin and Dalton, 2021). Therefore, examining the relative utility of different tools for reassurance and their substitutability (Most and Starr, 1984) also has important policy implications.

Another benefit of using a survey experiment is that it can directly measure the success and failure of alliance reassurance: the change in audiences’ perceived sense of security. One of the challenges to studying alliance reassurance is the lack of available data on reassurance success: while there are a few datasets on interstate crises in general and extended deterrence crises in particular (e.g. Brecher and Wilkenfeld, 1997; Huth, 1988), none of them contains data directly measuring reassurance outcomes. This is mainly because reassurance success is a perceptual variable, which makes efforts for systematic data collection extremely difficult. While public statements made by foreign policy elites can provide a picture of whether a US ally is successfully reassured, those statements do not always provide an accurate measure of those elites’ perceptions of security, because they are often designed to draw further concessions from allies, such as stronger commitments and additional force deployment. ⁷ Therefore, a survey experiment provides one way of addressing intractable problems of measuring reassurance success by comparing the perceived sense of security between those who are exposed to the treatments and those who are not.

In terms of mundane realism, a scenario-based survey experiment provides an adequate setting where respondents are likely to behave in a similar way in which they would evaluate the effects of US actions in real-world crises. Ordinary citizens tend to observe US military actions and official statements mostly through public media coverage (e.g. newspapers, online newspapers, social media). How those platforms convey the information about US actions is similar to how hypothetical scenarios in survey experiments deliver the information about those actions (e.g. summarized descriptions of an adversary’s behavior, US reactions, official statements of high-level policymakers, and visual aids describing those actions). In that sense, we can expect that respondents’ thought processes in a scenario-based survey experiment would be roughly similar to what they would experience in real-world events.

The sample

The experiment was fielded on 3,390 South Korean respondents in August 2022. ⁸ I contracted with a major South Korea-based polling research firm, Macromil Embrain, to field the survey via the Internet. Quotas were used to make the sample nationally representative across gender and age. ⁹

South Korea provides a useful case for studying alliance reassurance. As one of the key US treaty allies, Seoul is facing security challenges from North Korea’s tests of nuclear weapons and a range of missile capabilities, its military provocations, along with China’s dramatic expansion of its military capabilities. Specifically, North Korea’s development of its nascent nuclear capabilities and military actions had repeatedly triggered foreign policy crises for South Korea in the past, and its continuous efforts to augment both conventional and nuclear forces would remain a source of significant tensions with South Korea and the United States. Consequently, South Korea is one of the most likely cases where strong demands for US reassurance would arise. Understanding how the mass public of South Korea would respond to US attempts to reassure them could improve our understanding of the determinants of alliance reassurance outcomes.

Note that I do not argue that South Korea is a fully representative case of all US allies in different regions, given the important differences between the strategic environment each ally encounters. For instance, while North Korea, South Korea’s main opponent, possesses a relatively small nuclear arsenal, Russia, a key adversary of NATO member states, possesses the largest strategic and tactical nuclear arsenal in the world. These differences may lead to different responses from how other US allies respond to US reassurance efforts, such as nuclear signals. That said, empirical analysis of an East Asian ally can still make an important contribution, in part because existing experimental research mostly addresses US allies in Europe (Blankenship and Lin-Greenberg, 2022; Sukin and Lanoszka, 2024). Evidence from South Korean citizens can thus broaden the scope of empirical evidence in the growing literature on alliance reassurance.

Pretreatment questions

Figure 1 displays the flow of the survey experiment. Its design and survey questions build on previous works on crisis bargaining and alliance politics (e.g. Herrmann, 2017; Mattes and Weeks, 2019; Sukin, 2020; Weiss and Dafoe, 2019). After the consent process, respondents were first asked to answer a battery of pretreatment questions. ¹⁰

Figure 1.

The flow of the survey.

Scenario design and treatments

The survey used a hypothetical crisis scenario involving South Korea, the United States, and North Korea in 2025. ¹¹ After answering the pretreatment questions, all respondents were asked to read an introductory paragraph describing the background of the crisis. In this introduction, North Korea fires new models of intercontinental ballistic missiles (ICBMs) and medium-range ballistic missiles (MRBMs). North Korea announces that all tests were successful, and experts assess that these missiles could reach targets in the continental United States and South Korea. As a response, the United States and South Korea criticize North Korea’s tests and announce their plan to impose economic sanctions via the United Nations Security Council (UNSC). North Korea, in turn, declares that sanction imposition means an act of war and raises the readiness of its military forces.

This hypothetical crisis scenario closely resembles previous crises between three countries. For instance, North Korea’s tests of ICBMs triggered a crisis between the United States, South Korea, and North Korea in 2017. After Pyongyang’s second test launch of an ICBM, the UNSC imposed new sanctions against North Korea as a response. Rather than being an example of a unique, isolated event, the scenario represents a hypothetical but realistic situation that closely resembles recurring foreign policy crises on the Korean Peninsula. ¹²

Note that this background description contains initial responses from the United States. It is unlikely, if not impossible, that the United States would take no visible reactions to the adversary’s challenging actions that trigger a crisis, even though those actions may fall short of more serious, public signals for reassuring the ally. The joint statement that condemns North Korea’s missile tests and a public threat of sanction imposition, therefore, makes the background description a realistic control condition.

One might argue that a background condition that does not feature stronger US responses, such as reassurance signals that this paper examines, may not reflect how Washington typically behaves in crises. Yet, the background condition should not provide information on explicit reassurance signals, as it is intended to serve as a baseline against which the effects of those signals can be evaluated—the primary goal of this paper. Furthermore, in reality, the United States often employs reassurance signals selectively, rather than automatically sending them to allies. Recent research demonstrates that strategic calculations drive US attempts to reassure allies (e.g. Blankenship, 2020; Kudo, 2024). Therefore, the absence of more explicit reassurance measures in the background condition does not undermine its utility as a plausible baseline condition.

After reading the introduction, respondents were randomly assigned to either a control group or one of the four treatment groups. The control group did not receive any additional information. The first two treatment groups received the information that the United States deploys additional military forces, and they are participating in a joint military exercise with South Korean military forces, and these forces would stay until North Korea’s provocation ends. The first treatment group was told that one carrier strike group was deployed, while the second treatment group was told that two B-2 nuclear-capable bombers were deployed. The deployment of a carrier strike group and two B-2 nuclear-capable bombers represent US conventional military signal and nuclear signal, respectively.

The two military signal treatments capture multiple military actions, such as forward deployment and joint military exercises. While military deployments in these treatments are temporary (i.e. they are not permanent, although forces would remain in place until North Korea alters its behavior), moving additional forces closer to relevant theater, even if it is not permanent, can significantly influence the local balance of power during a crisis (e.g. Post and Sechser, 2024; Slantchev, 2005, 2011). As such, these treatments can engage the local balance of power mechanism (e.g. military signals reassure audiences by altering perceptions of the local balance of power), along with other theoretical mechanisms underlying the hypotheses above. Moreover, these actions are closer to actions providing significant fighting force, rather than “tripwires” forces (Musgrave and Ward, 2023; Schelling, 1966), in terms of their contributions to achieving desired battlefield outcomes or costly punishment. ¹³ Note that these treatments are designed to test the utility of different types of policy tools, such as different kinds of military forces (e.g. conventional or nuclear forces), rather than different types of military actions (e.g. mobilization or show of force) or different size of forces employed for military signals. These questions are addressed elsewhere (e.g. Blankenship and Lin-Greenberg, 2022; Post and Sechser, 2024).

In contrast to the first two treatments, the other two treatments provided respondents with information about non-military actions by the United States. The third treatment group received the information that the United States announces a diplomatic proposal, indicating its willingness to negotiate with North Korea on the issues of denuclearization and arms control if North Korea stops provocative behavior. This treatment represents US diplomatic overture toward North Korea. ¹⁴ The fourth treatment group received yet another piece of information, which describes the US public statement that its support for South Korea’s defense is unequivocal, and it will closely coordinate with South Korea to ensure readiness and deter further North Korean provocation. This information indicates US statements of support. ¹⁵

Note that the diplomatic overture statement does not detail other aspects of diplomatic initiatives by the United States (e.g. types of negotiations, the potential role of South Korea). This is because the goal of this treatment is to investigate the baseline reassurance effect of diplomatic overture toward adversaries. Future studies could explore whether variation in the content of diplomatic proposals has any heterogeneous effect on reassurance success.

In addition to the paragraph describing the details of each US action, each treatment also contains a visual aid depicting a key feature of the treatment (e.g. pictures of an aircraft carrier or a B-2 bomber for the military signal treatments, a spokesperson of US State Department for the overture and statement treatments). The design of these treatments closely resembles typical media reports on US military deployments and diplomatic statements, a key information channel through which South Korean ordinary citizens are likely to receive information on its security guarantor’s actions in a real-world crisis. ¹⁶

Dependent variables

As the key outcome measure, I used a question that asks, “If South Korea were to face this situation tomorrow, how safe would South Korea be from a potential North Korean military aggression?” The question uses the five-point scale, which ranges from “very unsafe” (1) to means “very safe” (5). As this paper defines alliance reassurance as convincing an ally that its security will be protected from external threats as promised, measuring the level of perceived safety from the adversary’s potential aggression in the scenario, instead of the perceived credibility of US security commitments, well approximates the concept of alliance reassurance. The question also builds on previous research’s survey measure of reassurance (e.g. Blankenship and Lin-Greenberg, 2022: 104–105), thereby promoting the accumulation of empirical knowledge.

As alternative measures of reassurance outcomes, I used two different questions. “If South Korea were to face this situation tomorrow, how trustworthy would the United States be as a security partner?” Finally, respondents were required to answer the following question: “If South Korea were to face this situation tomorrow, how valuable would U.S. security commitment to South Korea’s safety be?” These questions measure what respondents would also believe if their sense of security is increased by US actions. For instance, if a respondent is successfully reassured by US actions, then she is more likely to believe that the United States is a trustworthy security partner, and positively evaluate the contribution of American security commitment to South Korea’s security. With these two alternative outcomes questions, it is possible to test whether the key findings remain similar across different operationalizations of reassurance success. All three questions appear in a randomized order and use the five-point scale.

Data analysis

I first examine the effects of each treatment on alliance reassurance. Figure 2 shows the mean levels of perceived safety from North Korea’s aggression for the five groups. For ease of interpretation, the dependent variable is rescaled to vary from 0 (very unsafe) to 1 (very safe).

Figure 2.

The effects of US reassurance measures on perceived safety from North Korean military aggression.

Overall, respondents in the experiment took threats of military aggression by North Korea in the scenario very seriously. For instance, the average perceived safety of respondents in the control group is 0.382. Descriptions of North Korea’s missile tests and its military preparation in the experiment successfully made respondents concerned about South Korea’s safety from North Korea’s military incursion.

The results also demonstrate that military signals, both conventional and nuclear, and public statements of support increase the average level of perceived safety from North Korea’s military aggression. The average level of perceived safety is 0.424 in the conventional signal treatment group, 0.418 in the nuclear signals group, and 0.424 in the statement of support group. The differences from the control group are statistically significant at the 5 percent level.

In terms of substantive importance, the size of these treatment effects might appear to be modest. In terms of percentage points, the effect of the conventional signal treatment is 4.2 percentage points (0.042 out of 1); for the nuclear signal treatment, 3.6 percentage points; for the statement of support, 4.2 percentage points. These are not particularly small, however, compared to other experimental studies on similar topics. For instance, in her study on the effects of the credibility of US nuclear security guarantees on South Korean citizens’ support for an indigenous nuclear weapons program, Sukin (2020: 1020) finds that the effect of the treatment of credible nuclear guarantees is 5.86–8.26 percentage points, based on a binary measure of proliferation support.

On the contrary, the effect of the diplomatic overture treatment is not statistically distinguishable from zero. The average level of perceived safety in this group is 0.380, which is similar to that in the control group (0.382). These findings support hypotheses on the reassurance effectiveness of military signals (both conventional and nuclear) and the public statement of support for the ally while lending no support for the reassurance effect of the diplomatic proposal toward the adversary. This is in line with Sukin and Lanoszka’s (2024) findings that diplomatic summitry is the least preferred option by citizens of US allies in Central-Eastern Europe.

Overall, these key results remain similar when using two alternative measures as the dependent variable. An exception is the statement of support variable, which is still positively but no longer significantly associated with the dependent variable at the 5 percent level when the perceived trustworthiness of the United States is used as the dependent variable (p = 0.133). If we use the average of three outcome measures as the dependent variable, then the statement of support, the conventional, and nuclear signal variables are significantly associated with the dependent variable, while the diplomatic overture treatment has no significant effect. ¹⁷

Note that the lack of support for the hypothesis on the effectiveness of the diplomatic overture for reassuring allies is likely not the result of the failure of treatment administration. My analysis shows that the diplomatic overture treatment reduced respondents’ belief that North Korea would be concerned about US preemptive attack even if they do not launch aggression against South Korea (p < 0.000). This shows that the lack of the treatment effect of the diplomatic proposal treatment is not the result of its failure to manipulate respondents’ expectations about North Korea’s fear of preemptive attacks as intended. Such a shift occurs, but it is not translated into a positive evaluation of South Korea’s safety from North Korea’s military attack.

I also examine how each treatment shifted respondents’ expectations about how North Korea would evaluate a range of conditions when considering crisis escalation, such as US capability, resolve, and the likelihood of nuclear escalation and US preemptive attack. The conventional and nuclear signal treatments made respondents believe that North Korea would expect a greater chance of US intervention if they attacked South Korea and see US forces on the Korean Peninsula as more threatening. Respondents exposed to the nuclear signal treatment also had a stronger belief that North Korea would expect a greater chance of US nuclear retaliation and the occurrence of nuclear escalation. This shows that the nuclear signal treatment successfully changed respondents’ beliefs about the nuclear aspects of US intervention. Finally, all treatments except the nuclear signal made respondents more likely to expect that North Korea would believe a US preemptive attack is unlikely if they did not attack South Korea.

Interestingly, the statement of support has no measurable effect on respondents’ expectations of how North Korea would view US capability or resolve. Its effect on respondents’ belief of how North Korea would see the likelihood of US intervention is positive, but not significant at the 10 percent level (p = 0.132). It may be possible that while US statements of support lead respondents to believe Washington would come to the aid of Seoul if Pyongyang resorts to force (thus increasing their sense of security), they still might have some doubts about whether North Korea would also interpret it in the same way—a possible gap between the recipient’s and adversary’s interpretations. In the case of military signals, however, such a gap might be seen as smaller. Examining how citizens and policymakers of allies expect adversaries’ responses to US signals for reassurance and the effect of those expectations on the reassuring effect of different tools would be an interesting avenue for future research.

As displayed in Figure 3, respondents’ beliefs about North Korea’s expectations of US capability and the likelihood of its intervention also significantly mediated the effects of the conventional and nuclear signal treatments, except for the mediating effect of the belief of how likely North Korea would view US intervention would be on the conventional military signal treatment. ¹⁸ These findings imply that military signals, as argued by theories of interstate signaling, act as costly signals to credibly demonstrate senders’ capabilities and resolve.

Figure 3.

Beliefs about how North Korea would evaluate the likelihood of US intervention and US forces on the Korean Peninsula mediate the effects of the conventional and nuclear signals.

Overall, the results show that costly signals, such as military signals and statements of support, contribute to intra-crisis alliance reassurance. The breakout of a crisis does not make these instruments ineffective for mitigating allies’ security concerns. This suggests that ordinary citizens are “intuitive deterrence theorists” (Kertzer et al., 2021: 309). They appear to believe that demonstration of capability and resolve are effective for protecting their security as they would compel opponents to back down by raising the risk and costs of further crisis escalation and eventual military aggression. On the other hand, respondents were not persuaded that diplomatic overtures are an effective tool for preventing crisis escalation and protecting their security.

Why do diplomatic overtures fail to reassure South Korean respondents? The lack of the treatment effect does not necessarily mean that respondents simply reject core tenets of the security dilemma model, although it remains one possibility. Another explanation is that respondents may fear that Washington’s diplomatic overtures may lead to US–North Korea bilateral negotiations that prevent South Korea from having a voice in the negotiated settlement of the crisis. For instance, in the 1994 North Korean nuclear crisis, South Korean President Kim Young-sam was concerned that US–North Korea bilateral negotiations may exclude South Korea from influencing how the ongoing crisis is resolved, which has direct implications for South Korea’s security (Snyder and Lee, 2012: 171). ¹⁹ This fear of being sidelined may trigger another type of abandonment fear, which potentially reduces the effect of diplomatic overtures on alliance reassurance. Future studies might find heterogeneous reassurance effects of diplomatic toolkits, depending on how such diplomatic attempts are framed.

I also compare the relative utility of each policy for reassurance. US conventional military signal treatment does not have significantly stronger reassurance effectiveness than the nuclear signal or statement of support treatment (differences are 0.006 and −0.001; p = 0.643 and 0.954, respectively). However, it is significantly more useful than the diplomatic overture treatment (0.044; p = 0.001). Likewise, the nuclear signal treatment is significantly more effective than the diplomatic overture treatment for reassuring South Korean citizens (the difference is 0.038, p = 0.003), but the average level of perceived safety in the nuclear signal treatment group is not significantly different from that in the statement of support group (the difference is −0.007, p = 0.606). Finally, the statement of support treatment is also significantly more effective than US diplomatic overture treatment (the difference is 0.044, p = 0.001). Overall, these results indicate that several policy options create substitutable effects on alliance reassurance. For example, signals with nuclear forces as almost as effective for reassuring allies as conventional military signals. This challenges both overly optimistic and skeptical viewpoints on the role of nuclear weapons in alliance reassurance. In addition, public statements indicating Washington’s intention to support its ally could be as effective as the deployment of US military forces.

The effects of individual-level beliefs on reassurance

If reassurance success is in the eyes of the beholder, then individual-level beliefs could moderate the extent to which they are reassured by US policy instruments. I build on existing research and examine whether there are heterogeneous treatment effects across individual-level traits.

Studies find foreign policy orientation, which is “general approaches toward foreign policy that structure foreign policy attitudes” (Kertzer et al., 2020: 99–100), shapes individuals’ interpretations of signals and strategic situations (Brutger and Kertzer, 2018; Kertzer et al., 2020; Yarhi-Milo et al., 2018). ²⁰ My analysis finds that those having high scores on military internationalism, a general attitude toward the importance of military force in international affairs (Kertzer et al., 2020: 100), responded more positively to both military signals treatments. Interestingly, those high in cooperative internationalism, a belief about the value of cooperation with other countries to solve political issues in the international arena (Kertzer et al., 2020: 100), showed a lower average level of perceived safety when exposed to the diplomatic overture treatment. ²¹ These findings provide additional evidence that different individuals may respond to the same signal differently depending on their pre-existing views about how international politics work (e.g. Kertzer et al., 2020; Yarhi-Milo et al., 2018).

Second, I find that those who believed that North Korea harbors aggressive foreign policy aims felt much safer by the conventional signal treatment than those who believed that North Korea’s intentions are mostly defensive. This is consistent with existing findings that perceptions of the adversary’s intentions influence individuals’ preferred policy responses in crisis (e.g. Koopman et al., 1990). However, respondents’ perceptions of North Korea’s tendency to use force to achieve foreign policy aims have no moderating effects.

Several individual-level factors moderate the effect of the nuclear signal treatment. The level of respondents’ support for South Korea’s nuclear weapons development and whether they have military service experience positively moderate the reassurance effect of US nuclear signaling. However, those who held liberal political views felt less safe when exposed to the nuclear signal treatment. On the contrary, those who support the incumbent conservative party (People Power Party) felt safer by the nuclear signal treatment. Given that conservative party identification leads to support for an independent nuclear weapons program (e.g. Lee, 2023), this factor appears to make South Korean citizens particularly receptive to nuclear-based responses to security threats.

These findings suggest the potential need for a tailored approach to alliance reassurance. For instance, military signals can be particularly effective for reassuring a leader who strongly believes in military internationalism or has prior military service experience. If the support for an indigenous nuclear weapons program is high in a given ally, then nuclear signals could be especially a useful tool for reassuring the ally. It should also be noted, however, that recent research finds that credible nuclear security guarantees could also increase the support for nuclear proliferation further by increasing the ally’s need for greater control over nuclear decision-making (e.g. Sukin, 2020). Because most existing research assumes, rather than directly examine, the role of reassurance outcomes in shaping the association between nuclear guarantees and allied nonproliferation (Blankenship and Lin-Greenberg, 2022: 94), further research is necessary to examine the nature of the purported link between nuclear signals, reassurance, and nonproliferation outcomes and the risks and advantages of nuclear signals in reassuring allies.

Conclusion

What makes US allies reassured when they face immediate security challenges in foreign policy crises? How does the mass public in those allies respond to a range of US toolkits for alliance assurance? This paper answers these questions by using a scenario-based survey experiment with South Korean respondents. The results of my analysis demonstrate that US conventional and nuclear signals, as well as public statements of strong support, can boost respondents’ sense of safety from potential military aggression. However, Washington’s diplomatic overtures toward the adversary fail to produce similar assurance effects. In addition, military signals and statements of support have substantively similar effects.

What are the implications of these findings? To begin with, this paper shows that policy recommendations from mainstream theories of costly signals are also applicable in the context of intra-crisis alliance assurance. Existing research, both observational and experimental, tends to focus on testing hypotheses on costly signals in the context of the sender–adversary relationship (e.g. Fuhrmann and Sechser, 2014; Huth, 1988; Huth and Russett, 1984; Kertzer et al., 2020, 2021; McManus, 2017; Yarhi-Milo et al., 2018). My analysis shows that these results are also replicable in the context of the sender–ally relationship, even after the onset of intense crises.

It should be noted that this paper does not entirely reject the utility of diplomatic overtures for alliance reassurance and its logical foundations from theories of the security dilemma. First, in the experiment, respondents believed that Washington’s diplomatic overtures could significantly alleviate Pyongyang’s fear of US preemption, thus supporting a key argument of the security dilemma theory that strategies of reassurance could ameliorate the spirals of fear and mistrust (Jervis, 1976; Lebow, 1985; Stein, 1991). It is possible, therefore, that this expectation of the reduced fear of the adversary would be seen by US allies as helpful for escalation prevention if there are other facilitating conditions, such as settings for multilateral negotiations and credible promises of intra-alliance coordination. Second, diplomatic overtures may contribute to reassurance only if they are combined with confrontational costly signals, such as threats to use force. These possibilities are potential avenues for future research.

The findings of this paper also demonstrate the United States has a wide range of both military and non-military tools for reassurance. To begin with, public statements of support for the ally are perceived as carrying significant weight in the eye of the mass public of a key US ally. This suggests that without mobilizing or deploying military forces, US policymakers can substantially address the security fear of its allies, at least in the short term. This shows that Sukin and Lanoszka’s (2024) finding that reiterating security guarantees is preferred by citizens in European allies also applies to a different regional context. In the realm of alliance management, words can speak as loudly as actions. In addition, conventional military forces are as important as nuclear forces for achieving reassurance goals. Intra-crisis conventional military signals are as effective as peacetime conventional military deployment (e.g. Lanoszka, 2018) for successfully mitigating US allies’ security concerns. These results are in line with the call for increasing the role of non-nuclear capabilities for reassurance missions (e.g. Sukin and Dalton, 2021). Yet contrary to some skepticism on nuclear reassurance (e.g. Mount, 2023; Sukin and Dalton, 2021), my analysis shows that nuclear signals would remain an effective tool for alliance assurance. Therefore, it is likely that US policymakers would continue to consider nuclear capabilities an important toolkit for alliance management. In other words, conventional and nuclear capabilities are likely considered complements, rather than substitutes, to each other for reassuring allies in times of crisis.

My findings show costly signals, such as military actions and diplomatic statements of firm support, are effective tools for reassuring an ally involved in a foreign policy crisis. It does not necessarily mean, however, that signals that are useful for reassurance would also be effective for deterring adversaries. In principle, a signaling action could work differently vis-à-vis both allies and adversaries. For instance, while my analysis finds that joint exercises, combined with forward deployment, reassure allies, the literature on military exercises remains divided as to whether military exercises deter adversaries (e.g. Bernhardt and Sukin, 2021; D’Orazio, 2012; Kuo and Blankenship, 2022). Future research could advance the literature on interstate signaling by developing and testing a coherent theory of how a signal is perceived by different audiences and the conditions under which their interpretation converge (or differ).

Finally, it should not be taken for granted that the successful reassurance of one ally receiving reassurance signals always leads to a positive evaluation by all other allies of Washington’s reassurance efforts and its security commitments. As demonstrated by recent works (e.g. Henry, 2020; Kim and Simón, 2021), non-recipient allies, especially the allies not directly involved in the crisis or geographically distant, may want the United States to adopt restrained approaches, rather than sending firm and confrontational signals. As the United States and its allies are encountering security challenges from multiple opponents, such as China, Russia, North Korea, and Iran, future studies may explore what would be the effective mixture of policy options that can meet heterogeneous demands from different allies facing different security challenges.

Supplemental Material

sj-docx-1-ejt-10.1177_13540661251349606 – Supplemental material for Reassuring allies in times of crisis: assessing the effectiveness of tools for alliance reassurance in crises

Supplemental material, sj-docx-1-ejt-10.1177_13540661251349606 for Reassuring allies in times of crisis: assessing the effectiveness of tools for alliance reassurance in crises by Kyungwon Suh in European Journal of International Relations