This chapter assesses NATO’s intervention in Libya, Operation Unified Protector, which lasted from March 31, 2011, when NATO took over command and control of all military operations in Libya, until October 31, 2011, when the operation ended. The analysis focuses on the overriding questions of how and by whom collective action in the sense of military enforcement was taken; and on the related question of how international actors employing military force related to local actors in Libya. The chapter analyzes how NATO prepared for Operation Unified Protector and how the operation evolved. While the broad Security Council mandate provided NATO with a wide marge de manoeuvre, the military commanders had to face a number of crucial political predicaments related to the duration of the operation; the unspecified nature of the Security Council mandate; and questions of impartiality, neutrality, and regime change. Finally, the chapter examines the complex relationship between NATO and the United Nations, which had a decisive impact on the manner in which Operation Unified Protector was conducted – and assessed.