Necessity and Proportionality in the Law of War

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 remains controversial, both within each context and across contexts. This chapter aims to illuminate these controversies and to suggest how they should be resolved.

With respect to the law of force (the jus ad bellum), this chapter argues that necessity requires that states seeking to target non-state actors on the territory of another state must first seek the consent of the territorial state. Proportionality does not always permit states to use as much force as necessary to prevent or repel armed attacks, but instead requires that states balance the harm they expect to inflict against the harm they expect to prevent.

With respect to the law of armed conflict (the jus in bello), this chapter argues that military necessity does not authorize, justify, or otherwise provide a legal basis for acts of violence. Conversely, attackers must take precautions to avoid unnecessarily harming civilians, even at significant risk to themselves. Finally, an attack is proportionate only if it is reasonable to expect that it will prevent substantially more harm to attacking forces or to civilians than it will inflict on civilians.

Keywords: necessity, proportionality, war, armed conflict, precautions, consent