Alliances, Credibility, and Peacetime Costs

Alliances are not perfectly credible. Although alliances raise the probability of intervention into war, many allies do not honor their promise in wartime. A formal model of alliances as signals of intentions to explore the credibility of alliances is presented. One state threatens another. A third state shares an interest with the second in preventing the demands of the first. A simple model of a crisis among these three is solved first without an alliance between the second and the third states. The author then allows them the chance to form an alliance before the crisis. Alliances have two effects: (1) the ability of the allies to fight together is increased, and (2) peacetime costs on the allies are imposed. The model with alliances produces a wide range of strategic behaviors. The implications of the model for the formation, credibility, and deterrent effects of alliances are broadly consistent with stylized facts in the literature.