The first part of the just war framework is devoted to determining whether a resort to war—or intervention—is justified. War should be fought only for a justifiable cause of substantial importance. The primary just cause in an era of nations and states is a nation’s response to direct aggression, a central dictum solidified over the years but nonetheless problematic if one starts with St. Augustine’s insistence that it is better for the Christian to suffer harm than to harm another. That said, Augustine also recognized that statesmen (and women!) bore a responsibility for the wellbeing of their polities. Protecting citizens from harm is a fundamental norm, and it scarcely counts as protection if no response is made when one’s countrymen and women are being routed from their homes, hounded, slaughtered, and the like.
But there are other justified occasions for war. Aggression need not be directed against one’s own to trigger the jus ad bellum argument. The offense of aggression may be committed against a nation or a people incapable of defending themselves against a determined adversary. If one can intervene to assist the injured party, one is justified in doing so— provided other considerations are met. From St. Augustine on, saving “the innocent from certain harm” has been recognized as a justifiable cause: the innocent being those who are in no position to defend themselves. The reference is not to any presumption of moral innocence on the part of victims: nobody is innocent in the classic just war framework in that sense. This is another way in which the just war tradition guards against moral triumphalism: by insisting that, even though the balance of justice may fall more on one side than the other in cases of conflict, there should be no presumption that the aggressor is wholly evil; the aggressed against wholly innocent. Presuppositions of total innocence can and have fueled horrible things. In our time, this saving of the innocent is usually referred to as humanitarian intervention.
This does not mean, of course, that any one nation or even a group of nations can or should respond to every instance of violation of the innocent, including the most horrific of all violations—ethnic cleansing. The just war tradition adds a cautionary note about over-reach. Be certain before you intervene, even in a just cause, that you have a reasonable chance of success. Don’t barge in and make a bad situation worse. Considerations such as these take us to the heart of the so-called in bello rules. They are restraints on the means to be deployed even in a just cause. Means must be proportionate to ends. The damage must not be greater than the offenses one aims to halt.