I investigate the rationality of challenge and escalation when a third party, allied either to defender or challenger, is uncertain about the enemy’s power. The analysis illustrates how third-party ally power impacts general and immediate deterrence and willingness to intervene. In alliances with the defender, uncertainty about the enemy’s strength leads the third party to support the defender with a probability that decreases with the benefits that his intervention would provide although the likelihood that he is facing a strong challenger in war has increased. If the third party is allied to the challenger, ally behavior is more nuanced.
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