This introductory chapter discusses the importance of studying the process of coalition building. The puzzle of collective mobilization lies at the root of all politics. Studying the construction of multilateral military coalitions trains this puzzle on the context of international security—the one area of international cooperation that has traditionally been perceived as the most difficult to sustain a cooperative equilibrium. Moreover, the specific techniques used to build multilateral military coalitions affect how wars are fought. On the battlefield, coalition operations are supposedly more successful than non-coalition endeavors. Multilateral coalition building also affects the prospect for peace. Most peacekeeping deployments today are coalition endeavors, and research suggests that the stronger their participants, particularly in terms of personnel numbers and equipment, the more effective the missions are likely to be. Finally, coalitions unleash important socialization dynamics among participating states. They create common battle experiences and shape threat perceptions, military doctrine, and strategy for years to come. Sometimes, participation in a coalition can radically change a country's political trajectory. Thus, this book uses a social-institutional theory and evidence from over eighty multilateral military coalitions to explain coalition-building practices.