In 2004, Judith Gardam published her invaluable book, Necessity, Proportionality and the Use of Force by States. In it she makes the case that all lawful resort to military force at the international level requires compliance with the principles of necessity and proportionality. Prior to her work, discussions of resort to force often overlooked these crucial principles, perhaps in part because of a paucity of expert analysis. Professor Gardam's book addressed a critical deficit in our understanding of international law. Governments, scholars, and — most importantly — potential victims of armed conflict have been the beneficiaries.

In this contribution to the volume in her honour, I propose to revisit her discussion of necessity in the jus ad bellum in large part to respond to recent interest in characterising necessity as a lawful basis for the resort to armed force. This characterisation clashes with the position set out in Necessity, Proportionality and the Use of Force by States. The lawful resort to armed force must be based on at least one of two or possibly three exceptions to the United Nations Charter prohibition on resort to force. Necessity is not one of the exceptions. Rather, it is an additional condition for lawful resort to force, which applies once a recognised basis is established. A national leader ordering a military attack on the territory of another state must first confirm that there is a lawful basis for doing so and that the use of force is both necessary and proportional, and conforms to other relevant rules of international law, such as the law of state responsibility and human rights law.

The discussion begins with an overview of the general law on resort to force, then turns to a full focus on necessity. The principle of necessity is found in a number of international law categories with a distinctive meaning in each. Keeping the various meanings and histories separate is a challenge to scholars. In the area of resort to force, the principle of necessity is a restriction on force; it does not and cannot permit military force. In other categories of international law, necessity may provide a defence to otherwise unlawful action, such as imposition of a trade barrier, but not to the otherwise unlawful resort to military force.