It is easier to explain the absence of conflict between states than to explain how former rivals have eliminated war as a legitimate tool of statecraft. Kupchan's magisterial accomplishment, drawing on an extraordinary range of theories and cases, is to provide an overarching account of when and why countries in conflict move toward stable peace. Kupchan sees this as a sequential process in which Hobbesian anarchy must be addressed before crafting the institutions prescribed by John Locke, which, in turn, allow for deeper political integration. The conditions that Kupchan identifies as critical for reconciliation are institutionalized restraints on power, compatible social orders, and cultural commonalities -- factors he explores in a wide array of instances of both failed and successful peace building (from mere peaceful coexistence to full-fledged unification). His intriguing finding is that social and cultural affinities are a more important ingredient of stable peace between former enemies than are democratic governments or deep economic ties. This book will be read by scholars and policy thinkers for a very long time.