Major states have not gone to war against one another since their guns fell silent in 1945, the longest great-power peace since the seventeenth century. Building on this observation, this book argues that war in general is on the decline. “More wars are ending than beginning, once ended they are less likely to restart, and the remaining wars are more localized than in the past,” Goldstein writes. This good news, he contends, is a result of the international community “doing something right” in trying to tame war. Following the UN’s lead, peacekeepers, diplomats, aid agencies, and international organizations routinely intervene in troubled countries. The book narrates a number of UN-sponsored efforts to deal with various armed conflicts -- from the Suez crisis in 1956 and Congo in 1960–61 to post–Cold War UN peacekeeping failures and successes in countries such as Cambodia, Rwanda, and Somalia. The book works best as a portrait of the far-flung, underappreciated UN peacekeeping system as it has grown and evolved. But Goldstein does not provide a sustained explanation for why interstate war has declined. Neglecting to dig deeply into the changing sources of violence and insecurity, the book leaves its central puzzle unsolved.