Book excerpt: Why does the United States sometimes seek multilateral support for its military interventions? When does it instead sidestep international institutions and intervene unilaterally? In Coalitions of Convenience, 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, Coalitions of Convenience also considers the broader effects of power on international institutions and what the rise of China may mean for international cooperation and conflict.