What distinguishes a peace enforcement operation from an invasion? This question has been asked with particular vehemence since the US intervention in Iraq, but it faces all military operations seeking to impose peace in countries torn by civil war. This book highlights the critical role of international organisations (IOs) as gatekeepers to international legitimacy for modern peace enforcement operations. The author analyses five operations launched through four IOs: the ECOWAS intervention in Liberia, the SADC operations in the Democratic Republic of Congo and Lesotho, the NATO Kosovo campaign and the UN intervention in East Timor. In all these campaigns, lead states sought IO mandates primarily to establish the international legitimacy of their interventions. The evidence suggests that international relations are structured by commonly accepted rules, that both democratic and authoritarian states care about the international legitimacy of their actions, and that IOs have a key function in world politics.