This chapter examines the role of international law in preventing war and armed conflict. It begins by discussing three approaches to war and peace: the realist approach, the managerial approach, and the utopian visionary approach. It then considers some of the features of the United Nations system that were drawn from the League of Nations experience, including enforcement, dispute resolution, rule of law, prohibition of the use of force, and self-defence. The chapter also analyses how the UN Security Council deals with armed attacks undertaken by non-state actors, such as acts of terrorism. Finally, it outlines new challenges to the law on the use of force, particularly the new potential for armed conflict following the end of the Cold War, the issue of humanitarian intervention, and claims to enforcement of global community values.