As the United Nations passes its fiftieth anniversary, it has undergone a sea change in its approach to peacekeeping. Originally a stopgap measure to preserve a cease-fire, peacekeeping since the waning of the Cold War has become a means to implement agreed-upon political solutions to conflicts between antagonists. Placed inside war-torn states, UN peacekeepers have encountered new challenges as they oversee elections, protect human rights, and reconstruct government institutions. In this study, Steven R. Ratner offers a comprehensive framework for scholars, policymakers, and anyone seeking to understand this new peacekeeping. He sees the UN as an administrator, mediator, and guarantor of political settlements- roles that can conflict when peace accords unravel, as is all too common. He describes the numerous actors, inside and outside the UN, who are engaged in this process, often with competing interests. And in a historical review, beginning with the League of Nations, he reveals many striking precedents long before the 1990s.