War is among the most terrible of human realities. Yet Catholicism has never condemned all participation in war as morally impermissible. From the early Middle Ages to our own time, the church has consistently asserted that some evils or threats are so grave that they merit a vigorous armed response. “It is the other side’s wrongdoing that compels the wise man to wage just wars,” wrote Augustine, one of the earliest exponents of what is now termed “just-war theory.”

The intense debate over the U.S. invasion of Iraq, in which prominent neoconservative Catholics such as Michael Novak and George Weigel used just-war arguments to defend U.S. actions, has put the just-war theory under renewed scrutiny. This has called attention to the ongoing debate among just-war theorists about the correct starting point for moral reflection on war. Some thinkers, such as James Childress and Richard Miller as well as the United States Conference of Catholic Bishops, maintain that moral reasoning about war should begin with the imperative “Do no harm.” From this obligation, they argue, there derives a strong presumption against the use of force, a presumption that can be overridden only in very exceptional circumstances. By contrast, Rutgers scholar James Turner Johnson, Weigel, and others have argued for a more proactive stance. Moral thinking about war should begin, they say, with the duty of civic leadership to oppose grave wrongdoing. Its true starting point is a presumption against injustice.

On one side we have those who view participation in war as morally suspect. Military force should therefore be resorted to only in the most pressing circumstances. It should not be deemed part of the ordinary functioning of political leadership. Opponents of this view make the case that it dangerously underestimates the weight of evil in human affairs, hindering the ability of political leaders to counter it effectively. Significantly, these rival versions of just-war theory both appeal to Thomas Aquinas as a key source for their views.

Proponents of the “presumption against war” view maintain that Aquinas’s concept of just war is modeled on his more basic idea of legitimate defense. Hence they infer that the only correct rationale for resort to armed force is the purely defensive posture of repelling attack. Other reasons for “recurring to the sword” must be rejected as tantamount to aggression. Likewise, they endorse Aquinas’s admonition against being overly suspicious (“when a man, from slight indications, esteems another man’s wickedness as certain”). Relying on mere suspicions and not hard fact, statesmen can be tempted to condone preemptive attack. Finally, support is sought in Aquinas for the claim that the violent nature of military combat renders such action a poor instrument for the prosecution of justice. The point is not so much that the resort to lethal force is inherently wrong. Rather, it is held that warfare by its very nature is inclined to excess, and that such excess has been significantly aggravated by the destructiveness of modern weapons. Thus soldiers, even those whose intentions are good, inevitably get caught up in a spiral of violence.

In sum, the “presumption against war” interpretation places most of its emphasis on the prudential aspects of just-war reasoning. Issues surrounding proportionality and last resort occupy center stage. Principled matters of right-which earlier thinkers had treated under the rubrics of just cause and legitimate authority-are deemed to have only a secondary importance.

Proponents of the more robust version of just-war theory maintain that Aquinas never sought to limit the resort to armed force to simple self-defense. He did in fact think that occasions may arise when offensive war is warranted-to regain things wrongly taken, to thwart and punish organized evildoing, or to protect innocents from harm. Not only self-defense against actual attack (second use of force), but even a first use of force (offensive war) may be justified when it is the most efficacious response to wrongdoing. It has been argued for example that the Allies would have had good reason to initiate hostilities against Nazi Germany in 1936, when (in flagrant violation of his treaty obligations) Hitler remilitarized the Rhineland. On this understanding (as articulated by thinkers such as Aquinas, Vitoria, Suarez, Grotius and others in the classical just-war tradition), the distinction between defensive and offensive force does not reduce to the distinction between just and unjust war. To the contrary, limiting force to strict defense would have the undesirable effect of paralyzing action in the face of an unjust status quo or a dangerous military threat. In this interpretation, just cause remains the paramount consideration. Matters of prudential concern (proportionality, last resort, and so forth) should be subordinated to deliberations about just cause.

ikewise, on the question of preemptive strikes, advocates of “presumption against injustice” argue that Aquinas’s admonition against being overly suspicious does not present his last word on the topic. For he also asserts that “when we have to apply a remedy to some evil, whether our own or another’s, in order for the remedy to be applied with greater certainty of a cure, it is expedient to take the worst for granted.” Read in the light of our concern about weapons of mass destruction, this statement would seem to countenance a strategy of preemptive action.

Finally, one would be hard put to find Aquinas agonizing over the charge that military action inherently tends to excess. Nowhere does he emphasize the special moral dangers attendant on an active engagement in war. In fact, Aquinas seems more worried about the spiritual dangers endemic to the life of the businessman than about the moral risks of the military profession. To devote one’s life to business has, in his words, “a certain debasement attaching thereto” for “it satisfies the desire for gain which knows no limit and tends to infinity....Trading is open to many vices.” Significantly, no such reasons are advanced in his earlier treatment of just war, wherein he alludes to the nobility of the military calling. In response to the concern about the indiscriminate destructiveness of modern war, a case can be made that new precision weaponry enables military professionals to be more effective at shielding noncombatants.

I have emphasized the differences separating the two versions of just-war theory, but it should not be forgotten that they also have much in common. Both stand opposed to the doctrine of raison d’état in which war is viewed as simply another way to advance the national interest. Like Aquinas, neither side would deny that there is a “presumption against war” if this means that war necessarily requires justification. Whenever punishment is meted out, it must first be established that it is in fact merited. In this respect war stands apart from those acts of governance-giving speeches, setting up schools and hospitals, holding elections, reaching trade agreements-that do not presuppose prior wrongdoing by another party.

It was precisely this understanding that offensive force can be justified only as a reaction to wrongdoing that led Hugo Grotius, the most systematic of all the classical just-war theorists, to issue a strong caution against preventive war. Grotius conceded that the specter of a future attack might legitimate a preemptive action if it could be shown that the danger was immediate and certain. Still, he also insisted that “fear of an uncertainty cannot confer the right to resort to force.” Using armed force preventively, solely to eliminate an adversary’s ability to inflict future harm, he deemed illicit. It would punish crimes not yet committed, perhaps not even planned, and would make fear a principle of action in international relations, thereby opening a Pandora’s box of anticipatory first strikes. “That the possibility of being attacked confers the right to attack is abhorrent to every principle of equity,” he wrote. “Human life exists under such conditions that complete security is never guaranteed to us.”

Finally, among just-war theorists, there is a general consensus that any party resorting to armed force must take steps to protect innocent bystanders. Since it is inevitable that some such harm will occur in wartime, this risk must be factored into any decision to use armed force. For the decision makers themselves war ought to be a source of sorrow. “War has no place among the useful arts,” concluded Grotius. “Nay,” he added, “it is so horrible that only the utmost necessity, or true charity, can render it honorable.”

That is the traditional Catholic teaching. Yet it is precisely this sense of sorrow at war, this understanding that offensive war requires special justification (since it presupposes clear and determinable wrongdoing by the other party), that is conspicuously absent from the war rhetoric of the neoconservatives in the Bush administration. In the run-up to the Iraq invasion, Dick Cheney, Donald Rumsfeld, and Paul Wolfowitz consistently spoke as though there was a clear-cut presumption in favor of war. War to disarm Saddam Hussein was the default position from which all other options (inspections, for example) were to be judged. Hence the burden of proof was shifted from the advocates of the invasion to those who dared question its necessity. This alone should make us pause before too quickly assuming a direct line of continuity between the neoconservative strategy of preventive military intervention and even the most robust traditional teaching on just war.