Necessity and Proportionality in International Law

Abstract

International law constrains both the resort to military force and the conduct of hostilities. In both contexts, principles of necessity and proportionality limit the lawful use of lethal violence. The content of these principles is controversial, however, both within each context and across contexts. This chapter aims to illuminate these controversies and suggest how they should be resolved. International lawyers often refer to the legal norms constraining the resort to military force as the jus ad bellum or jus contra bellum and to the legal norms constraining the conduct of hostilities as the jus in bello . Infelicitously, philosophers use the same terms to refer to the moral norms to which these legal norms quite imperfectly correspond. To minimize confusion, I refer to the first set of legal norms as the law of force and to the second set of legal norms as the law of armed conflict (although many refer to the latter as international humanitarian law ). The Law of Force The primary source of the contemporary law of force is the United Nations Charter, a multilateral treaty to which almost all states are parties. Many experts contend that the UN Charter's provisions regulating the use of force reflect jus cogens , that is, peremptory norms of customary international law from which no derogation is permitted. The UN Charter requires states to settle their international disputes by peaceful means and prohibits states from using military force on the territory of other states. This general prohibition has three narrow exceptions. First, the UN Charter does not prohibit the use of force with the consent of the territorial state, although such force may be constrained by other legal norms. Second, the UN Security Council may authorize member states to use military force on the territory of other states if necessary to maintain or restore international peace and security. Finally, states retain an inherent right to use military force in individual or collective self-defence if an armed attack occurs, until the UN Security Council takes measures necessary to maintain international peace and security.