This study assesses the rules of international law relevant to the use of force against non-State actors. The rules of international law on the use of force are the lynchpin of the project of international law for a more secure and peaceful world. Yet, as important as they are, the rules of international law on the use of force are also highly contentious. With the shift in the nature of conflicts from inter-State wars to conflicts involving non-State actors, and with the growth in the threat of global terrorism, the focus of the law on the use of force has shifted to the use of force against...