The Effective Power of Military Coalitions: A Unified Theoretical and Empirical Model

We argue that the effective power of a coalition depends not only on its members’ raw capabilities but also how much effort they exert. Free riding and competition between partners mean a coalition’s power is not the sum of its members’ power. We develop a unified model of crisis bargaining and war-fighting between coalitions. Using data on the escalation and outcomes of international disputes, we structurally estimate the model parameters to identify determinants of countries’ force multipliers and audience costs. We find that demographic and economic characteristics are the most important determinants of military effectiveness, with regime type and geopolitical considerations playing lesser roles. The structural model also allows us to simulate counterfactual outcomes in disputes between coalitions. This allows us to test historical claims, such as the claim that earlier American involvement would have made a substantial difference in both World Wars, even in the face of strategic free riding by its allies.