The Inherent Right to Self-Defence and Proportionality in Jus Ad Bellum

While force used by a state in self-defence must meet the demands of proportionality there is confusion over the meaning of the term in this, jus ad bellum, context. One source of confusion lies in the existence of two competing tests of proportionality, the 'tit for tat' and the 'means-end' tests. Since the legality of unilateral use of force by a state depends on the legitimacy of its aim – self-defence against an armed attack – the 'means-end' test would seem more appropriate. However, there is no agreement over the legitimate ends of force employed to achieve this aim. Is the defending state limited to halting and repelling the attack that has occurred, or may it protect itself against future attacks by the same enemy? May a state that has been attacked use force in order to deter the attacker from mounting further attacks? The 'means-end' test of proportionality rests primarily on the necessity of the means used to achieve legitimate ends. Disagreements over proportionality are in this context usually really disagreements over those ends. While the appropriate test in this context is generally the 'means-end' test, in some cases, such as use of force in response to a limited armed attack, the 'tit for tat' test of proportionality might be more appropriate. Finally, I show that little attention has been paid in the jus ad bellum context to the 'narrow proportionality' test, which assesses whether the harm caused by the force outweighs the benefits to the state using that force. The apparent reason for this is the assumption that this question is only relevant in jus in bello. I argue that while necessity of the force used is indeed the main issue in jus ad bellum, there is still place for assessing narrow proportionality.