This chapter considers the law relevant to the question whether, and if so in what circumstances, states are entitled to resort to the use of force under international law as a response to acts, or threats, of international terrorism. The legality of the use of force between states under international law is referred to as the ‘jus ad bellum’. Part A of the chapter addresses key aspects of the relevant legal framework, which part B then analyses alongside examples of state practice in response to international terrorism since 9/11. Specifically, it addresses the military interventions in Afghanistan and Iraq which followed 9/11 and the on-going targeting of members of al Qaeda and associated groups around the world.

The distinction between the body of law addressed here, and those considered in other chapters of this study, bears emphasis at the outset. The jus ad bellum which determines when use of force on another state’s territory is lawful must be distinguished from jus in bello that encompasses the rules that apply once force has been used and a conflict is underway, and which applies irrespective of whether the resort to force was lawful. The lawfulness of the use of force between states, discussed here, is also distinct from the lawfulness under human rights law of the use of lethal force. The use of force may be lawful under the jus ad bellum, but still a violation of the individual’s rights under the quite different normative standards of IHRL. As practice will show, confusion – whether deliberately fuelled or inadvertent – has often surrounded these divergent areas of law and the justifications available to states under each.