Abstract

Suppose that state A attacks state D without warrant. The ensuing military conflict threatens international peace and security. State D (I assume) has a justification for defending itself by means of military force. Do third parties have a justification for intervening in that conflict by such means? To international public lawyers, the well-rehearsed and obvious answer is “yes.” Threats to international peace and security provide one of two exceptions to the legal and moral prohibition (as set out in Article 2[4] of the UN Charter) on using force as a means for resolving interstate disputes. Just war theorists are not as verdictive. Compared to the ethics of humanitarian intervention and the ethics of national self-defense, the ethics of third-party military involvement in interstate conflicts remains underdeveloped in contemporary just war theory. This essay begins to fill that gap. I argue that to defend such interventions is tantamount to defending preventive military force, deterrent military force, and the resort to force in more cases than standardly thought. I then provide an account and limited defense of the deterrence argument. I show that deterrence is morally justified in relatively few cases and examine two problems with the argument: deterrence failures and the level of uncertainty under which leaders who use deterrent force operate. I conclude that we should take seriously the possibility that nonintervention, construed as the rejection of the direct use of military force, is the morally correct response to the most serious threats to international peace and security.