An Introduction to Public International Law

This chapter introduces the jus ad bellum: the rules of law determining when states may resort to war or, more broadly, the use of armed force. These rules must be distinguished from the jus in bello: these are the rules of law that apply in armed conflict (known as international humanitarian law). In order to put the current jus ad bellum rules into perspective, the chapter begins by introducing the concept of collective security and demonstrating how this was applied during the League of Nations era. The following sections set out the relevant rules of the UN Charter on the prevention and regulation of recourse to the use of force, with a particular focus on the prohibition on the threat or use of force. The chapter also discusses the collective use of force, meaning the use of force authorized by the Security Council, and the unilateral use of force in self-defense. Finally, the chapter examines whether new exceptions to the prohibition of the use of force are emerging -- in particular, humanitarian intervention and the responsibility to protect.