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. But 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 strikingly under-developed in contemporary just war theory. This paper makes a start at filling the gap. It argues 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. It then provides an account and limited defence of the deterrence argument. It shows that deterrence is morally justified in relatively few cases, and examines two problems with the argument: the problems raised by deterrence failures, and the problems raised by the level of uncertainty under which leaders who use deterrent force operate. It ends by concluding that we should take seriously the possibility that non-intervention, 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.