The Limited Reach of Ad Bellum Proportionality

There is wide agreement that although ad bellum proportionality (ABP) does not constitute a legal constraint on waging war, it constitutes a moral one, implying that a state ought to refrain from waging an otherwise justified war if it estimates that the war will be disproportionate. Against this view, I argue that the same considerations that tell against regarding ABP as a legal rule tell also against regarding it as a moral one; (a) as a matter of fact, such a rule would be ignored and (b) if it is not ignored, it would significantly undermine deterrence. In addition, there seem to be insurmountable epistemic difficulties in establishing ex ante whether a war will be disproportionate or not. I conclude by arguing that this understanding of ABP is best captured within a contractarian view of the ethics of war. It is mutually beneficial for all international players to agree on a rule that exempts them from the requirement to make sure that the (defensive) wars they wage are ad bellum proportionate.