Military alliances are a constant feature in international politics, and a better understanding of them can directly impact world affairs. This book examines why alliances endure or collapse. As a distinctive feature, it analyses asymmetric alliances focusing on the junior allies' decision to continue or terminate a military agreement. It deepens our knowledge of alliance cohesion and erosion, investigating the relevance of the weaker side's preferences and behavior in alliance politics. The author examines the literature on alliance persistence and termination and puts forward a theoretical model that helps interpret historical and contemporary cases in a way that is useful for expert researchers and non-expert readers alike.