Today, United Nations peacekeeping is the multidimensional management of a complex operation, usually following the termination of a civil war, designed to provide interim security and assist parties to make those institutional, material, and ideational transformations that are essential to make sustainable. That is a new role for the UN. UN operations during the Cold War were more limited and focused on monitoring or policing the adherence to a truce by hostile parties.This new, expanded role for the UN represents an effort to respond to complex new challenges to international security that have emerged since the end of the Cold War. An explosion of new internal armed conflicts led to a similar explosion in UN peacekeeping missions in the mid-1990s. The UN's new perspective on how to build after civil war is embodied in two landmark reports-the Brahimi and No exit without strategy reports of 2000 and 2001 that built on Secretary General Boutros Boutros-Ghalis 1992 report Agenda for peace and its 1995 supplement. While the UN has been generally effective in its new role, important and highly publicized failures have generated policy debates on how to improve the UN's capacity.This article engages with those policy debates by analyzing the record of UN peacekeeping and peacebuilding. Peacebuilding involves a blend of several intervention practices, including mediation, observation, policing, tactical enforcement, conflict resolution, humanitarian assistance, reconstruction, and institutional transformation-all helping to create peace. Much criticism of UN missions in the popular press or policy literature is based on a claim that goals are not sufficiently adaptive to local contexts and interventions by different actors sometimes have conflicting effects. Different intervention practices are interdependent in complex ways: while one form of intervention may help shore up the foundations for another intervention strategy, the two together may work better or, in some cases, they may work well only if they are properly sequenced. Appropriate standards of success may also vary by context and by the proximity to the war. If the goal of intervention is social justice and political inclusion, then best practices will be different than in cases where the goal of intervention is simply the absence of war.For any conflict situation, sustainable peace is the best measure of successful peacekeeping. Efforts to achieve that measure are influenced by three key factors that characterize the environment of the postwar civil peace: the degree of hostility of the factions, the extent of local capacities remaining after the war, and the amount of international assistance provided. Together, these three constitute the interdependent logic of a peacebuilding triangle: the deeper the hostility, the more the destruction of local capacities, the more you need international assistance to succeed in establishing a stable peace.This article will explain how the dimensions of the triangle affect the nature of the postwar challenge and how the UN has been able to help countries transition from war to peace. We will focus on our preferred standard of success: the achievement of what we call participatory peace-a that includes not only the absence of war, but also restoration of the state's sovereignty over all of its territory and some degree of political openness. Resolving problems of divided sovereignty is an essential part of state-building that UN or other actors cannot afford to ignore. UN missions can have positive and lasting effects by keeping the in the early stages of the process, when risks of a return to war are greatest.1GENERATIONS OF UN PEACE OPERATIONSIn the early 1990s with the end of the Cold War, the UN's agenda for and security rapidly expanded. …