Military coalitions and the outcome of interstate wars

Approximately one-third of all interstate conflicts are multilateral, with the majority of these having a coalition of states fighting on at least one side. Despite the frequency of coalition wars, coalitions have not received much attention within the conflict literature. This paper presents the first general study on the effectiveness of coalitions during interstate wars. While there are many drawbacks to fighting as part of a coalition, the benefits of cooperation outweigh the cost, making coalitions more likely to win wars. An empirical examination of war outcomes between 1816 and 2007 confirms this hypothesis; coalitions have greater odds of victory than states fighting outside a coalition. This finding holds after controlling for possible endogeneity and selection bias.