What Drives Support for Armed Humanitarian Intervention? Experimental Evidence From Dutch Citizens on International Law and Probability of Success

Abstract

Under what conditions do individuals support armed humanitarian intervention (AHI) in situations where mass atrocities are ongoing? This article tests several hypotheses about support for AHIs to isolate and interact two potential drivers: international law and the probability of success. It leverages an original, pre-registered experiment from a (quota) representative sample of over 1500 Dutch citizens. Consistent with our hypotheses, we find that support for AHI increases when an action is authorized by the UN Security Council (UNSC) and has a high (80%) chance of success. But the Dutch remain supportive of AHI in situations of mass atrocities even when AHI has a low chance of success (20%). Importantly, we find that the chance of success does not affect the support for AHIs as much as the international law does. This suggests that in similar situations legal and procedural reasons may influence public opinion more than a logic of consequences.

Keywords

Public opinion armed humanitarian intervention international law just war theory probability of success mass atrocities human rights United Nations Security Council

Under what conditions do individuals support armed humanitarian intervention to attempt to stop ongoing mass atrocities? Armed humanitarian intervention (AHI) is any foreign military intervention without the target state’s consent, the main purpose of which is protecting civilians from mass atrocities (Pattison, 2010, 24–30; Frowe, 2016, 87). In some circumstances, international legal authorization for AHI may be in tension with moral and prudential considerations. For instance, NATO intervened militarily in Kosovo in 1999 without UN Security Council (UNSC) authorization, which some argued was “illegal but legitimate” (Goldstone and Tham, 2001, 4). How do individuals balance respect for international law and the probability of protecting innocents from mass atrocities? Building on a growing body of evidence about what drives support for AHIs (Boettcher, 2004; Jentleson and Britton, 1998; Kreps and Maxey, 2018; Kreps and Wallace, 2016; Maxey, 2020; Wallace, 2019), we contribute a pre-registered survey experiment¹ using a factorial design to manipulate two potential drivers of public support for AHI simultaneously: international law and the probability of success. Our design allowed us to test both the main and interaction effects on public willingness to support intervention with a representative Dutch sample of >1500 participants. Testing the interaction between legality and chance of success in the context of hypothetical mass atrocities in a foreign country constitutes our main contribution to the debate.

We found more support for AHIs when they were legal and had a high (80%) chance of success, compared with any other condition. Furthermore, even in scenarios of an illegal AHI with low (20%) probability of success, on average participants still supported AHI. This level of support, in combination with the small (insignificant) difference in support in scenarios with no information about legality and information that the intervention was illegal, suggests that an intervention might be considered justified or legitimate (seen as the right thing to do), even if it is illegal in light of the international law.

Experimental evidence shows that politicians and lawmakers tend to align their policy choices with public opinion (Chu and Recchia, 2022). Therefore, understanding better what drives public support for AHI is important because public opinion can influence choices of politicians in situations where they are considering the use of force (Tomz et al., 2020).

Theoretical framework and hypotheses

There are at least three ways that individuals can reason about why to support or abstain from supporting a war: instrumental, moral, and (international) legal justifications (Dill and Schubiger, 2021; Kreps and Maxey, 2018). Instrumental reasoning means attempting to achieve one’s aims most effectively (Dill and Schubiger, 2021, 614–16). Moral constraints are typically discussed (in the West) in the just war theoretic tradition (Frowe, 2016; McMahan, 2009; Walzer, 1977). The international legal aspect we focus on here is UNSC approval for an AHI.

In our study, we focus on international legal constraints on the use of force and compare those with a moral or prudential consideration: probability of success. Part of the goal of this study is to isolate the independent and interaction effects of a legal justification and a moral or prudential one in situations of ongoing mass atrocities. Theoretically, we aim to improve our understanding of why individuals might support contributing financially, materially, or soldiers to an AHI, when their home country is not threatened.

Our contribution differs from others’ work in several ways. First, we focus only on AHI. AHI differs from other types of war because it requires humanitarian goals, namely, to prevent mass atrocities. This differentiates the sort of use of force, and context, from other types of war such as self-defense and wars waged for national interests, narrowly defined. Second, the intended beneficiaries are foreign nationals, not fellow citizens. Important work has been done on how individuals tradeoff attempting to protect foreigners and fellow citizens (Boettcher, 2004; Johns and Davies, 2019; Pratto and Glasford, 2008; Sagan and Valentino, 2017, 2020). We isolate the potential influence of self-defense by stipulating there is no threat to fellow nationals. We differ from the important work of Kreps and Maxey (2018) because they vary the framing of reason for military action, namely, humanitarian reasons of protecting civilians or upholding state sovereignty after a foreign power invaded another (such as Russia with Ukraine), whereas we hold constant the reason for intervention. In all our vignette scenarios, the perpetrators and victims are domestic actors in foreign countries.

Moreover, whereas Kreps and Maxey (2018) assess morality by asking whether the United States has a moral obligation to intervene, we focused on the probability of success of any AHI. In our vignettes, success is portrayed in terms of the chance of the AHI protecting large numbers of innocent civilians from mass atrocities in the next 6 months. This operationalization of success follows a definition of the primary objective of AHI, namely, prevention of imminent deaths (Seybolt, 2007). Using this definition of success allows us to present a tenable estimate of success while maintaining ecological validity by not stating a concrete number of civilians saved.

UN Security Council authorization (international law)

Existing findings point to the importance of institutional approval for intervention in the eyes of citizens (Chapman, 2007, 2009, 2011; Chapman and Reiter, 2004; Grieco et al., 2011; Tago and Ikeda, 2015; Wallace, 2019). The approval of the UNSC signals the permissiveness to use force to the public (Chapman and Reiter, 2004) and increases support among those who exhibit lower confidence levels in the foreign policy decisions of their national leader (Grieco et al., 2011). Furthermore, in times of UNSC dissensus, the vetoing actor’s international identity sometimes influences public opinion. In one study of Japanese citizens, which country vetoed a UNSC decision to intervene mattered for the support of the intervention (Matsumura and Tago, 2019), whereas in another it did not if the vetoing power were Russia or China (Tago and Ikeda, 2015, 406). We focus on the letter of the law instead of testing support depending on which country votes against a resolution. For this reason, we did not name which country, or countries, vetoed the resolution. Importantly, we are interested in how large of a role international law plays when compared with various probabilities of success. Finally, although a few countries such as the United Kingdom have argued that in rare situations where just war theory criteria are met UNSC authorization may not be required for an AHI to be legal (Newman, 2021), we rely on the mainstream position that UNSC authorization is necessary for the legality of AHI.

The probability of success (moral/prudential reasons)

Just war theory (JWT) is the leading framework in the West to debate the morality of war (Frowe, 2016; McMahan, 2009; Walzer, 1977). One jus ad bellum consideration, the probability of success, holds that war is only permissible if there is some reasonable likelihood that a just cause will be achieved. A justification for this principle is because leaders send others to fight (and risk death) on their behalf, and it is impermissible to do so if there is an insufficient chance of success (Frowe, 2016, 60).

Scholars have found this precept can influence public opinion. The beliefs about the success were found to have a strong association with the support for military interventions in a study of Dutch citizens (Mazepus et al., 2022). A survey experiment based in the United Kingdom found that public’s initial support for intervention motivated by moral considerations can become eroded by concerns regarding the intervention’s effectiveness (Davies and Johns, 2016). The public’s lack of belief in the effectiveness of military humanitarian interventions was found to be largely shaped by their perception of questionable success in achieving political objectives in past interventions (Davies and Johns, 2016). While some studies varied the probability of success in terms of targeting perpetrators (Sagan and Valentino, 2017), others framed success in terms of the ratio of intervening soldiers’ survival to death ratios (Boettcher, 2004) or the likelihood of success in destroying a perpetrators’ physical target (Sagan and Valentino, 2017; Tomz and Weeks, 2020). We focus on the probability of success in protecting large numbers of innocent civilians from mass atrocities. The probability of success could be framed in moral or instrumental/prudential logics, or both. We do not take a position on that here because we think there are valid reasons it could be framed either way, and our goal was to test its effects on public opinion compared with international legal justifications.

Hypotheses

Following the literature on the drivers of support for international humanitarian interventions, it is not clear whether the international legality of an intervention increases the support for interventions or if illegality decreases it. It is also possible that both effects are in place at the same time. Therefore, we test the following hypotheses about the effects of information about legality and illegality on public support for AHI—a type of military intervention—by comparing them to the level of public support when such information is not provided (control condition: No information).

H1a

Information that the United Nations Security Council authorized the intervention increases public support for military intervention.

H1b

Information that the United Nations Security Council did not authorize the intervention decreases public support for military intervention.

Support for international military interventions may be limited by the public perception of unsuccessful past interventions. Hence, it is questionable to what extent estimates regarding intervention success alter support for intervention—if lower estimates further decrease support, if higher estimates increase support, or if both or neither applies. Accordingly, hypotheses on the effects of information about low and high intervention success are tested to investigate public support for intervention when receiving success estimates in contrast to support when success estimates are not specified (control condition: intervention success difficult to determine).

H2a

Information about higher chance of success increases public support for military intervention.

H2b

Information about lower chance of success decreases public support for military intervention.

Third, based on the aforementioned findings, we hypothesize that:

H3a

The effect of legality on support for intervention is smaller when the chance of success is low.

These hypotheses may be less obvious than they first appear. Given how abhorrent mass atrocities are, perhaps support for AHI would not vary with chance of success (at least when operationalized as 20% or 80%) because even a modest or low chance is worth taking to save the lives of innocent civilians. Some may support AHI for feeling that their country should do something, even if the action is unlikely to have a positive outcome. This might result from an “action bias” (Patt and Zeckhauser, 2000), a tendency to focus our attention on the action in question (e.g., trying to help victims) rather than an unintended consequence (e.g., the chaos that eventually followed the AHI in Libya.) Similarly, perhaps some think the approval of the UNSC should not matter because mass atrocities generate special responsibilities which might override institutional constraints. At least 104 countries have supported a French and Mexican lead proposal to urge the P5 not to veto resolutions in situations of mass atrocities (Global Centre for the Responsibility to Protect, 2015). Interestingly, Tago and Ikeda (2015, 406) find that if China or Russia vetoes a UNSC resolution, public support of Japanese citizens for a US-lead intervention does not significantly decrease compared with a successful resolution. This suggests that which countries veto a UNSC resolution might matter more than whether an intervention is technically legal, at least among Japanese citizens when there is a strong public perception that something needs to be done.

Research design and context

We conducted a survey experiment using a 3 (legality) × 3 (chance of success) between-subject factorial design. Participants were randomly assigned to read one of nine scenarios that varied the legality (no information about the legal status, legal, or illegal intervention) and the chance of success (uncertain, low, or high) of a hypothetical AHI. If a respondent failed a manipulation check, we prompted them to re-read the vignette. If they failed a second time, we allowed them to move on (see S5 and S8).

The scenario offered an explanation that the legality is derived from the approval or disapproval of the five permanent members of the UNSC. This conception of legality corresponds with existing studies which experimentally varied legality in terms of UNSC authorization to test its effect of support for intervention (Matsumura and Tago, 2019; Wallace, 2019). We varied the probability of success by stating that it was uncertain, or that there was a 20% (1/5) or 80% (4/5) chance of success in protecting innocent civilians (see experimental vignettes in S3). We measured the support for the AHI with five questions tapping into different aspects of support that formed a reliable scale (see S4).

We selected the Netherlands to conduct this survey for several reasons. First, there is little research examining the public opinion of the Dutch on topics related to public support for AHIs. Second, the Netherlands is a mid-sized NATO member that has historically contributed to various AHIs and peacekeeping operations aimed at protecting civilians, such as contributing F-16s to the 2011 AHI in Libya. Some Dutch military operations have had large and lasting impact on Dutch politics, politicians, and perhaps public opinion. Foremost among these are the historic failures of a Dutch contingent of UN peacekeeping mission in Bosnia in 1995, which resulted in a genocide (Power, 2002, chap. 11). The reverberations of these failures echoed for years in the Netherlands. The Dutch government resigned in 2002 following the release of a critical report. In 2019, the Dutch Supreme Court ruled that the Dutch were partially responsible for the death of hundreds of these civilians. Third, participation by states such as the Netherlands may lend legitimacy to AHIs led by countries such as the United States. Understanding better why individuals in states such as the Netherlands are willing to support AHIs may matter for when larger and more powerful states are able to garner international support for AHIs.

Results

We collected a quota-based sample of 1606 Dutch citizens in June 2022, using the panel provided by the agency Flycatcher. For detailed demographics, comparison to the population, and balance checks for the random assignment across conditions, see S6 and S7. We have implemented manipulation and comprehension checks (see S5) and conducted our analyses with the full sample of participants (reported here) and with the sub-sample of participants who answered all three checks correctly at their first attempt (without the prompt to re-read the vignette, reported in S8). The results are robust to sample specification.

We tested our hypotheses with full factorial ANOVA (F(8, 1581) = 4.72, p < .001, η2 = .02). The results show significant main effects of legality (F(2, 1581) = 12.90, p < .001, η2 = .02) and chance of success (F(2, 1581) = 6.24, p = .002, η2 = .01) and no significant interaction effect (F(4, 1581) = 0.07, p = .99, η2 = .00).

Regarding legality, we find support for H1a: participants in the condition signaling legality expressed higher level of support for AHI (M = 4.95, SE = 0.06) by comparison with both participants who received no information about legality (M = 4.64, SE = 0.06, Cohen’s d = 0.21) and participants who received information about illegality (M = 4.51, SE = 0.06, Cohen’s d = 0.30). Because support for AHI did not differ significantly across the conditions with no information and information that the AHI is illegal (t(2, 1588) = 1.48, p = .140, Cohen’s d = 0.09), we do not find support for H1b.

Regarding the chance of success, we find support for hypothesis H2a: participants who read about an intervention that has a high chance of success expressed higher level of support for AHI (M = 4.87, SE = 0.06) than participants who read about an intervention with uncertain chance of success (M = 4.66, SE = 0.06, Cohen’s d = 0.14) and participants who read about an intervention with a low chance of success (M = 4.56, SE = 0.06, Cohen’s d = 0.21). Here too, because support for AHI did not differ significantly among participants who received information about low intervention success and participants who received information about uncertain intervention success (t(2, 1588) = 1.10, p = .809, Cohen’s d = −0.06), we find no support for H2b. Figures 1 and 2 illustrate these results.

Figure 1.

Predictive marginal means with 95% confidence intervals of support for armed humanitarian intervention across legality (left panel) and chance of success (right panel) conditions.

Figure 2.

Violin plots of support for armed humanitarian intervention across conditions of chance of success (low, uncertain, and high) and legality (illegal, no information, and legal). The white dot shows the median, the dark blue bar shows the interquartile range, and the vertical line extends to the highest and lowest value. The violin shows the estimated kernel density.

As the difference between illegality and no information conditions was insignificant (see S9 for pairwise comparisons), the default assumption that citizens may have had was that interventions were illegal (or that illegality is not actually signaling a norm transgression). No significant difference between low and uncertain chance of success in turn might indicate that, on average, the default assumption is that AHIs have a rather low chance of success. We found no significant interaction between legality and chance of success. The effect of legality is not smaller when the chance of success is low by comparison to high (Figure 2). Given our samples size for the relevant comparison (n = 713), we had sufficient power (> 90 %) to detect a significant interaction (α = .05) if the effect of legality in the high chance condition was of medium size (d = 0.5) and in the low chance condition it was absent (d = 0). We did not, however, have sufficient power (50 %) to detect a significant interaction if the effect in the high chance condition was of medium size (d = 0.5) and in the low chance condition it was small (d = 0.2).²

Implications

These findings lend support for the idea that public opinion regarding AHI is influenced both by international law and probability of success. This suggests that both procedural (legal) and moral or prudential considerations matter for public opinion formation. Importantly, on average individuals in the sample were generally rather supportive of AHI in all the scenarios we presented, indicating that interventions to prevent loss of innocent lives are seen as justified. This means that while legal authorization from the UNSC and probability of success play some role in support for AHI, these are only two of multiple factors that shape public opinion.

There are several limitations to our study. First, other factors such as risks to soldiers, financial costs, cues from elites and peers, and which country vetoes a UNSC Resolution may influence support for AHI (Jentleson and Britton, 1998; Kreps and Maxey, 2018). Second, probability of success is only one dimension of successful interventions. Therefore, this study should not be taken as a comprehensive evaluation of what drives support for AHI. Rather, its aim is to contribute to the relative weight individuals put on international law and probability of success. Third, to detect smaller interaction effect, future studies would need to replicate the experiment with a larger sample. Fourth, some research has found that respondents are more willing to support the use of force against fictitiously named countries in vignette experiments (Majnemer and Meibauer, 2023). Repeating the experiment with naming a real country could address this concern.

Importantly, we find that both the chance of success and international law affect the support for AHIs. The legality, however, has a somewhat larger effect. Our results show that cues of legality are important in shaping public opinion about AHIs, but also that on average Dutch citizens are willing to respond to mass atrocities even if the chance of success is not high or the intervention not approved by the UNSC.

Supplemental Material

Supplemental Material - What Drives Support for Armed Humanitarian Intervention? Experimental Evidence From Dutch Citizens on International Law and Probability of Success

Supplemental Material for What Drives Support for Armed Humanitarian Intervention? Experimental Evidence From Dutch Citizens on International Law and Probability of Success in Honorata Mazepus, Eamon T Aloyo and Natália Kubalová in Research & Politics.

Footnotes

Acknowledgments

We are grateful to Jan Aart Scholte and the Global Transformations and Governance Challenges (GTGC) programme at Leiden University for providing a seed grant which funded this research. We thank Joris Larik for the grant and assistance with various aspects of the paper, Femke Bakker for help with translation, Florian van Leeuwen for his feedback, and the participants of the GTGC Conference 2023 and the GTGC group for their comments on the earlier drafts of the manuscript. Finally we appreciate the helpful feedback from the anonymous reviewers and the editor, which improved our paper.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Global Transformations and Governance Challenges (GTGC) programme at Leiden University.

ORCID iD

Honorata Mazepus

Supplemental Material

Supplemental material for this article is available online.

Notes

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