Third-Party Participation in Wars

This article investigates the calculus of third-party participation in ongoing wars. Altfeld and Bueno de Mesquita have shown that under an expected utility model of third-party participation, the probability of a nation's joining is in inverse proportion to the ex ante probability of other nations' joining. In seeming contradiction Yamamoto and Bremer have argued that one major power's decision to enter a war increases the probability that other powers will do likewise. In this article it is shown that these two perspectives, in fact, do not contradict one another. It is found that a nation is more likely to join a war when the initial belligerents in the conflict have few formal alliances, and when, despite ex ante beliefs, many other nations have already joined in the war. It is also shown that a third party is more likely to join the war in support of a friend than in opposition to a foe.