Does Just War Doctrine Require Moral Certainty?

Pope Leo XIV has made it clear that the U.S. war on Iran does not, in his judgment, meet the criteria of just war doctrine. Timothy Broglio, archbishop for the U.S. Military Services, has said the same, as have several other American bishops. I have argued that the war’s violation of just war conditions is manifest. That conviction has only been strengthened in the month since, as the Trump administration has made evident that it had no feasible plan to avoid plunging the world into entirely foreseeable economic chaos, has deliberately targeted civilian infrastructure, and even threatened that “a whole civilization will die” if demands are not met.

Some Catholics disagree with this peremptory judgment. But the more sober among them, while making a case for the war, acknowledge that the conflict is problematic. For example, Rusty Reno identifies both the possible advantages and the potential downsides to military action, and concludes that “it is unwise to issue confident moral judgments” about the war. Robert Royal says he’s “not sure” whether the war is just, but appears to think that those who “already claim to know” ought to “suspend judgment.” Francis X. Maier defends the war but acknowledges that it is “debatable” whether it is just.

These arguments imply that just war doctrine allows one to support a war as long as some plausible argument can be given for it, even if not a conclusive argument. This is not the case. What has long been the standard teaching in the Catholic just war tradition is that the probability of a war’s being just is not good enough. The case for the justice of a proposed war must be morally certain. Otherwise, it is morally wrong to initiate the conflict. It follows that, if writers like Reno, Royal, and Maier acknowledge that the case for the war is debatable at best, then they should oppose the war.

Let’s look first at what the tradition says, and then at why it says it. There were, in the early modern history of just war theory, some thinkers who defended the view that a war could be morally defensible as long as it was probably just. Francisco Suárez took such a view, but it soon lost out. St. Robert Bellarmine, a Doctor of the Church whose teaching on matters of politics has been especially influential within Catholic theology, was among those who argued instead that: “The cause of war should be neither trivial nor doubtful, but weighty and certain, lest, perchance, the war bring about more harm than the hoped-for-good, hence if there is any doubt . . . the ruler himself sins, without doubt; for war is an act of retributive justice, but it is unjust to punish any one for a cause not yet proved.”

Similarly, St. Alphonsus Liguori, another Doctor of the Church and the prince of moral theologians, teaches: “There is no doubt that since war generally brings in its train so many evils and so much harm to religion, to innocent people, to the honor of women, etc., in practice it is hardly ever just if declared on probable reasons of justice alone and not certain reasons.”

The twentieth-century scholar of natural law John Eppstein, summing up what became standard Scholastic doctrine, wrote:

This school of thought—saving Suarez, whose dangerous opinion of probable justice was soon rejected—agrees upon the certainty of a just and sufficient cause as alone justifying recourse to war. . . . Nor will any reader who examines the trend of Catholic thought . . . and most of all the practice and preaching of the Papacy in recent times, be left in any doubt that this body of neo-scholastic philosophy is attune with the true mind of the Catholic Church.

The work of prominent early twentieth-century Catholic natural law theorists confirms this consensus. For example, Heinrich Rommen writes: “The justice of the cause must be certain. The authority that declares war must be certain of its right and of the objective justice of its cause. . . . Impartiality is a most difficult moral task. And this difficulty increases if, instead of full certainty, only a probability about the justness of the casus belli can be reached.”

Similarly, Johannes Messner held that “in view of the possibly far-reaching consequences of modern war . . . it is evident that nothing less will do than a high degree of moral certainty about the justice of the cause.” John K. Ryan, in Modern War and Basic Ethics, writes that “in modern times it is more imperative than ever that a nation be certain that it is justified in entering upon a course of war.”

It is important to emphasize that these are works from before Vatican II and therefore cannot be accused of reflecting postconciliar theological mushiness or liberalism. Indeed, the great hero of Catholic traditionalism Alfredo Cardinal Ottaviani went so far as to suggest in the late 1940s that under modern conditions, the criteria for a just war must be so strict that it is doubtful whether any war could meet them. I believe this goes too far, and further than what became the standard view. But it illustrates how strongly the tradition emphasizes moral certainty.

This view was well represented in standard pre-Vatican II manuals of moral theology. For example, in their Moral Theology: A Complete Course, Fr. John McHugh and Fr. Charles Callan write that “the government may not declare war, unless it is morally certain that right is on its side . . . one should refrain from hostilities as long as one’s moral right is uncertain.” Fr. Austin Fagothey’s Right and Reason holds that “not only must the nation’s cause be just, but it must be known to be just . . . one must be sure of a just cause before fighting.” When recent popes have held proposed wars to a very high standard—a standard they have often said the wars failed to meet—they were by no means innovating, but simply reflecting what had long since become common theological teaching.

Exactly what degree of certainty is required, and why does the tradition require it? Both questions are best answered by way of stock examples. Suppose a hunter considers firing into some bushes. On standard natural law thinking, he may do so only if he is certain there is no other hunter behind them. If he considers it merely probable that there is no one behind them and fires anyway, he is guilty of wrongdoing, even if he doesn’t hit anyone. For his action was reckless. Or consider a jury deciding whether to sentence an accused murderer to be executed. They may not do so if they think it merely probable that he is guilty (as opposed to being certain “beyond a reasonable doubt” that he is guilty). For to leave open the serious possibility of executing an innocent man would be a grave injustice, just as the murder itself is.

This is the degree of certainty that the tradition says governing authorities must have about the justice of a war before initiating it. The reason should be obvious. If it is gravely immoral to risk killing a fellow hunter or a person wrongly accused of a crime, then it is even more gravely immoral to enter into a war that is, at best, only arguably just.

To forestall misunderstandings, note that the claim is not that governing authorities must have absolute or metaphysical certainty (of the kind we have when we know, for example, that 1 + 1 = 2). Nor does the tradition claim that we need to have certainty about every aspect of a war. We need to be morally certain only that a proposed war meets all just war criteria (just cause, lawful authority, right intention, right means, and so on).For example, one of the criteria of a just war is that “there must be serious prospects of success” (as the Catechism puts it). Hence, governing authorities don’t need to be certain of the success. However, they do need to be certain that there are serious prospects of success.

In her essay “War and Murder,” Elizabeth Anscombe adds another important consideration. Pacifism, she argues quite firmly, is not a morally serious position. At the same time, she notes, war affords occasions for evildoing that go well beyond those that arise in the course of ordinary police work. For one thing, war has a special tendency to stoke passions of the kind associated with pride, malice, cruelty, and self-righteous overconfidence in the justice of one’s cause. For another, it involves an unusually high number of occasions in which innocent lives might be put at risk, in which combatants will be tempted to put those lives at risk, and in which they might realistically get away with doing so. Consequently, though there can be such a thing as a just war, in practice, Anscombe judges, “wars have mostly been mere wickedness on both sides” and “the probability is that warfare is injustice.”

Thus does the tradition require moral certainty if a war is going to be just. Those who admit that the case for the Iran war is problematic are wrong to conclude that Catholics can legitimately disagree over the matter, or that we should suspend judgment. If the war does not meet the standard of moral certainty, then we can be certain that we must oppose it.