Exiting the Coalition: When Do States Abandon Coalition Partners during War?

When do countries abandon coalition partners during war? Prominent arguments about alliance dissolution focus on peacetime, yet the ability of alliances to influence international politics ultimately hinges on their cohesion or dissolution during war. In this article, I argue that battlefield circumstances heavily influence the likelihood of defection from coalitions. First, countries fighting independently from their partners make attractive candidates for wedge strategies and hence are more likely to defect. Second, coalitions are more likely to collapse when their members see victory in the overall war as less likely. These predictions contrast with common expectations from theories of the balance of power and of collective action. I test hypotheses about wartime developments statistically using new time-varying data on both front-level troop contributions and battle deaths. Consistent with theoretical predictions, countries are more likely to abandon coalition partners if fighting alone and when the coalition fares worse in recent fighting.