Missions in Somalia, Liberia and Sierra Leone were all part of an important and lasting post-Cold War peacekeeping trend. Peacekeepers have attempted to support, and in some instances even replace, governments in providing security and, ultimately, restoring the monopoly on the use of force in countries coming out of civil war. While multilateral organizations seek to establish sustainable peace in countries around the world, we still do not fully understand why they succeed or fail at this critical task. State authority deters violence by holding a monopoly on the legitimate use of force and wielding it so that individuals and groups refrain from engaging in serious conflict. How can we expect outsiders to play this essential role when the state is too weak to do so?