A Theory of Jus in Bello Proportionality

The law of armed conflict prohibits attacks ‘which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated.' The importance of this legal prohibition, commonly referred to as the principle of jus in bello proportionality, is difficult to overstate. While the precautionary principles regulate how armed forces may pursue particular military advantages, the proportionality principle regulates whether a particular military advantage may be pursued or must be abandoned. Even if attacking forces select weapons, tactics, and targets that best avoid or most reduce harm to civilians, even at their own risk, they must forego one path to victory if the expected civilian losses are too great. Rather than inflict excessive harm on civilians, attacking forces must find another way to win. An account of jus in bello proportionality must satisfy two apparently conflicting demands. First, such an account must explain how we can rationally compare civilian losses with military advantages. At the same time, such an account must apply symmetrically to all parties to every conflict and independently of the jus ad bellum legality or morality of any party’s overall war effort. Existing accounts of jus in bello proportionality satisfy either one demand or the other. In this chapter, I offer a new account that satisfies both demands. Along the way I hope to answer a number of additional questions that any serious account of jus in bello proportionality must address. I argue that an attack that inflicts harm on civilians is jus in bello proportionate only if it prevents substantially greater harm to the attacking force or its civilians over the remainder of the conflict. This account of jus in bello proportionality does not compare incommensurable and imprecisely comparable values, only immediate losses to civilians and future losses to civilians and to attacking forces. In addition, this account applies symmetrically to all parties to an armed conflict, independently of the jus ad bellum morality and legality of their use of military force. Attacks that are disproportionate under this account are morally impermissible when carried out by just combatants, and disproportionate attacks carried out by unjust combatants are morally worse than proportionate attacks carried out by unjust combatants. It follows that both just and unjust combatants have decisive moral reasons to avoid attacks that are disproportionate under this account, and the law would guide soldiers well by prohibiting such attacks.My account of jus in bello proportionality is in one way more determinate than existing accounts — for example, it does not compare incommensurable values but instead compares immediate losses to civilians and future losses to civilians and to attacking forces. At the same time, my account must still grapple with the predictive uncertainty inherent in determining whether immediate losses inflicted will be redeemed by future losses avoided. Accordingly, I explore a number of decision procedures and rules of engagement that officers may use to make the best possible decision given the limited information available to them.