The just war tradition traces its inception from St. Augustine's great City of God. Augustine argued that the vocation of the statesman may well require the use of force. Force is a feature of a fallen world. But Augustine contrasted war entered into reluctantly with Rome's opportunistic wars of aggression. According to Augustine, a decent order permits simple tasks to flourish in relative peace--the tasks of raising families, succoring the weak, educating the young, worshipping God. None of these can flourish if chaos, disorder, and violence prevails.

Over the centuries, and from this humble if powerfully cast beginning, just war teaching evolved into a complex doctrine. It is a tradition that relies on prudential judgments on a number of critical matters. Today, most importantly, its teaching is urged upon relevant decision-makers, those charged with the solemn tasks of making the determinations about whether, and when, a resort to force is justified.

There are critics of the just war tradition who claim, wrongly, that it is mere window-dressing--so many nice sounding nostrums behind which is little more than crude realpolitik. They mistakenly assume just war theory exists to justify any war a nation decides to fight. In fact, the tradition challenges precisely any use of force that is reckless or misguided.

For pacifists, of course, war is never justified; the complex discriminations of just war tradition are beside the point altogether. Other critics insist that the just war tradition may have worked well in previous eras, but has outlived its usefulness. It is, quite simply, out-of-date. These critics hold that, given today's weapons technologies, the rule of discrimination-requiring armies to distinguish combatants from non-combatants-can't possibly be met.

Some go further, claiming that the word "justice" itself can be given any meaning. Each side in a conflict naturally thinks its cause is just, goes this argument, and there is really no clear way to adjudicate between such subjective claims. It is impossibly old-fashioned, in this view, to believe that one can actually make a determination about what is more just, or less unjust, in the contemporary world.

There are decisive answers to these criticisms. Let's begin with the question of judging justice. The just war tradition is clear about the elements that go into assessing a just cause, or casus belli, of compelling force. It begins with a response to a direct act of aggression or the imminent threat of such. Other jus ad bellum considerations include right authority (private persons are not authorized to make war, only public authority can do that); last resort (everything short of war need not have been tried, but it must have been considered); and probability of success (don't barge in and make a bad situation worse.) Clearly, all these issues remain relevant to the present moment and to the Iraq situation specifically.

Let's look at the criterion of imminent threat more closely. Who is in danger of being harmed? What is our responsibility if a threat is imminent? On this score, modern just war thinking is deeply indebted to the traditions of international law, which assume a world of sovereign nation-states. The violation of a state's sovereignty is the test of a threat or a harm: Am I the victim of aggression that violates my sovereignty? This was the case of Iraq's 1991 invasion of Kuwait. Kuwait, as a member of the United Nations, was able to evoke the United Nations Charter that bases membership--and the crime of aggression--on state sovereignty.

But just war teaching also makes provision for what some have called "anticipatory self-defense," that is, the right to defend oneself if one possesses reliable knowledge that, unless one moves to preempt a threat now, there is a high probability that one could be subjected to a devastating blow later. That `later' must be imminent--not projected into the future by decades. Assessments of imminent threat will be based not only on what sorts of weapons a potential aggressor has, but on his past history. Does that history include unprovoked attacks on others? Does it include the use of weapons that do not, by definition, discriminate between combatants and non-combatants. And so on.

These matters are never simple, particularly so if you add St. Augustine's argument that the obligation of charity means a war may be occasioned by a charge to protect the innocent. The innocent are not limited to one's own nationals-certainly not within Christian just war teaching, which does not privilege sovereignty as a value above all other political values. The obligation of caritas extends beyond the boundaries of one's polity to encompass the innocent in other countries if they are being brutalized, murdered, terrorized, or threatened by overwhelming or despotic force.

This is the question British Prime Minister Tony Blair has been asking repeatedly in recent months: Is it moral to permit the intense human suffering and the terror of living under one of the world's most brutal regimes to continue? Is there no obligation to remove the dread hand of tyranny if one has the means to do so, and if the harm that comes from an armed effort to disarm Saddam will in all likelihood be less than the palpable, continuing harm of allowing him to remain in power?