Why does the United States sometimes seek multilateral support for its military interventions? When does it instead sidestep international institutions and intervene unilaterally? In a comprehensive study of US military interventions in the post-Cold War era, Sarah Kreps shows that contrary to conventional wisdom, even superpowers have strong incentives to intervene multilaterally: coalitions confer legitimacy and provide ways to share the costly burdens of war. Despite these advantages, multilateralism comes with costs: multilateral responses are often diplomatic battles of attrition in which reluctant allies hold out for side payments in exchange for their consent. A powerful state's willingness to work multilaterally, then, depends on its time horizons—how it values the future versus the present. States with long-term—those that do not face immediate threats—see multilateralism as a power-conserving strategy over time. States with shorter-term horizons will find the expediency of unilateralism more attractive. A systematic account of how multilateral coalitions function, also considers the broader effects of power on international institutions and what the rise of China may mean for international cooperation and conflict.