Introduction

This chapter considers the role of necessity and proportionality in the delimitation of the use of force by States up to the adoption of the United Nations Charter in 1945. In relation to unilateral State action, the requirements of necessity and proportionality in ius ad bellum find their only current expression in the context of self-defence against an armed attack. These principles, however, have a long history associated with the history of the regulation of the resort to force over the years. Proportionality in particular has played an integral role in the development over many centuries of theories restraining violence. Although the content of the equation has differed widely over the years, the idea that there should be some equivalence between means and ends is a consistent theme of debates over licit and illicit force. During the Middle Ages, proportionality operated both as a limit on the resort to arms and to some extent as a general restraint on the conduct of warfare, albeit without a great deal of definite content in the latter context. Such limitations were derived from the view that disproportionate violence was both unnecessary and undesirable and combined aspects of what is found today in ius ad bellum and international humanitarian law (IHL).

Necessity, in the sense that war is by way of last resort when other means have failed to achieve the object, is inherent in much of just war theory.