As followers of Jesus, who taught us to love our enemies, the editors of Plough are committed to Christian nonviolence and roundly reject just war theology as incompatible with the gospel. That said, we recognize that just war theory is intended to restrain war and reduce killing, and we welcome any discussion that might help lead humankind away from the scourge of war in general and the insanity of nuclear deterrence in particular. Since the war in Ukraine has again raised the specter of nuclear war, we asked the Catholic philosopher Christopher Tollefsen how Catholic just war theory applies to nuclear deterrence. His response follows.

The encounter of just war theory with the question of nuclear weapons may perhaps have begun in earnest in Somerville College, Oxford, in spring 1956. It is a well-known story now, told most recently in Benjamin Lipscomb’s fine book The Women Are Up to Something. The Catholic philosopher Elizabeth Anscombe was attempting to block Oxford from presenting former president Harry Truman with an honorary degree. (Lipscomb’s book takes its title from what the dons were saying about her efforts: “The women are up to something at Convocation … we have to go and vote them down.”)

Anscombe related her objection in her essay “Mr. Truman’s Degree”: “For men to choose to kill the innocent as a means to their ends is always murder, and murder is one of the worst of human actions.” But by “worst of human actions,” Anscombe did not simply mean and therefore to be weighed against others of the worst human actions in order to decide which course of action is preferable. Were that the case, any action might be “‘morally good’ no matter how objectionable the thing done may be.” Rather, Anscombe held that morality, including in warfare, identifies classes of action, such as murder, that are simply to be excluded from practical consideration altogether. Thus the norm against deliberate killing of the innocent, understood in war, Anscombe said, as those “who are not fighting and those who are not engaged in supplying those who are with the means of fighting,” identified that act as simply not to be done.

Anscombe, and other Catholic thinkers such as John C. Ford, SJ, held that Truman’s decision to unleash atomic weapons on the Japanese cities of Hiroshima and Nagasaki could only be understood as murder: the intention was clearly to kill many Japanese civilians as an inducement to Japanese surrender. Did that decision save lives? Anscombe’s answer to that question was nuanced; she thought the continuation of the war depended in part on an unreasonable Allied demand for unconditional surrender. But ultimately the question was irrelevant, for moral absolutes are to be followed regardless of the consequences.

The secular philosopher Thomas Nagel has credited Anscombe with returning just war theory to twentieth-century philosophical discussion. But the tradition of thought from which she drew dates back at least to Augustine, and was articulated clearly by Aquinas and subsequent scholastics, as well as Protestant thinkers such as Hugo Grotius and, more recently, Paul Ramsey. The tradition has focused on two sets of moral requirements. The first identifies the conditions under which it is morally permissible to go to war (ius ad bellum): there must be just cause, right intent, competent authority, probability of success, the exhaustion of non-warring alternatives, and the prospect that the goods to be achieved will be proportionate to the evils incurred.

Of primary concern here are the second set, which establishes the conditions under which fighting may be justly carried out (ius in bello). These are the “principle of discrimination,” that only combatants are to be intentionally attacked; and the “principle of proportionality,” that no greater force be used in achieving properly military goals than is necessary. This principle can be extended to encompass collateral damage upon noncombatants, harms that are not intentionally inflicted. Such harms may be permissible but may not be disproportionate to the military gains sought.

Just war theory has been responsible, over the course of its history, for genuine advances in attention to the moral character of war, but in the twentieth and now twenty-first centuries, it has struggled with two great in bello questions concerning nuclear weapons. One is retrospective, though its implications are forward-looking. Was, as Anscombe claimed, the use of nuclear weapons in World War II morally impermissible? The other is of abiding concern in a world containing approximately fifteen thousand nuclear weapons: Is nuclear deterrence morally permissible, even if one believes that the use of nuclear weapons is morally abhorrent?

The second question has been especially important to Catholic moral philosophers and theologians who agree with Anscombe’s verdict. For while Catholic moral teaching has been unequivocal regarding the unacceptability of using nuclear weapons, it has been less so about nuclear deterrence until recently. The change has come, perhaps unsurprisingly, with Pope Francis. “If we also take into account the risk of an accidental detonation as a result of error of any kind, the threat of their use, as well as their very possession, is to be firmly condemned,” Francis said in 2017, and he and his spokesmen have reiterated this view since.

The just war tradition is part of a larger tradition of theological and philosophic thought: the natural law tradition. While the Catholic Church holds that it can teach both doctrine and morals without error when guided by the Holy Spirit, it also teaches that its moral teachings may be known through the light of reason and are in that sense common to all reasonable and mature people. They are “written on the heart” of the human person. The Catholic Church’s view is thus that its developing teaching on nuclear weapons converges with what may be learned from natural reason and natural law. Sound just war thinking and Catholic teaching will, the church holds, agree at the end of the day.

So how does one get from Anscombe’s principle, that murder is not to be done, to Francis’s proscriptions not only on the use, but the threat of use, even the very possession, of nuclear weapons?

Catholic teaching on the principle of discrimination is clear. In Gaudium et spes, the Second Vatican Council declared: “Any act of war aimed indiscriminately at the destruction of entire cities or extensive areas along with their population is a crime against God and man himself. It merits unequivocal and unhesitating condemnation.”

One sees here the application of Anscombe’s principle: there must be no intentional killing of the innocent. In 1983, in a pastoral letter titled The Challenge of Peace, America’s National Conference of Catholic Bishops extended this analysis. Use of nuclear weapons was illicit, they wrote, on either of the two principles of ius in bello. If, as argued by Anscombe, Fr. Ford, and others, the bombing of Hiroshima and Nagasaki involved the deliberate killing of noncombatants, such acts were ruled out by the norm against intentionally killing the innocent. But even if the deaths of those many thousands were not intended, but a side effect (collateral damage) of a genuinely military mission, the bishops indicated that this would still have been a violation of the principle of proportionality.

Yet they rendered a somewhat different conclusion about nuclear deterrence: “Deterrence is not an adequate strategy as a long-term basis for peace; it is a transitional strategy justifiable only in conjunction with resolute determination to pursue arms control and disarmament.” Clearly, a strategy that is even temporarily justifiable is not absolutely morally impermissible. For the Catholic Church’s view on moral absolutes is that they admit of no exceptions and are to be complied with always and everywhere; they are not ideals to be reached for eventually, but moral necessities.

This verdict of the American bishops reflected an earlier, albeit highly tentative, claim of Pope John Paul II that deterrence “as a step on the way toward a progressive disarmament, may still be judged morally acceptable.” The acceptability of deterrence was embraced by conservative Catholics such as George Weigel and Michael Novak, who were rightly aware of the threat posed by the Soviet Union and believed that nuclear deterrence was the only way to preserve Western liberties. Unequivocal condemnation of deterrence appears not to have been part of mainstream Catholic just war thinking, though it was, of course, a mainstay of pacifist groups such as Pax Christi and Plowshares.

A major turning point in the dialectic over nuclear deterrence occurred in the late 1980s, however, with the publication of Nuclear Deterrence, Morality and Realism by John Finnis, Joseph Boyle, and Germain Grisez, three of the most eminent Catholic moral thinkers of the latter half of the century. Finnis, Boyle, and Grisez advanced an argument in some ways simple and straightforward, yet powerfully grounded in just war theory and echoing core Catholic teachings.

Their argument begins with the principle of discrimination: it is immoral to deliberately kill noncombatants. To this, they add the plausible claim that it is immoral to intend what it is immoral to do. Consider: if it is wrong for you to cheat on an exam, it is also wrong for you to intend to cheat, even if, as it turns out, the exam is canceled or you are prevented from taking it. Thus, if it is immoral to deliberately attack noncombatants, it is wrong to intend to do so.

Finnis, Boyle, and Grisez argue that the intention to commit a wrong is equally wrong even when the intention is conditional. Such conditional intention would be wrong even if the intended act was one you hoped not to carry out: A bank robber may sincerely hope that no security guard appears, while nevertheless intending to shoot him dead if he does.

Finnis, Boyle, and Grisez argue that the nuclear deterrent strategy pursued by the United States was in fact structured upon the conditional intention to kill noncombatants. The goal of the policy of deterrence was to prevent a Soviet attack; the means of preventing it the threat of unacceptable losses, including, ultimately, unacceptable losses on civilian population centers. If we assume that the threat was not a bluff (a possibility that the authors considered and rejected), it is clear that it involved a conditional intention to do what was impermissible and is therefore morally impermissible.

Reflection on the bishops’ 1983 pastoral letter suggests a way of extending the trio’s argument. Even if threatened nuclear reprisals do not involve deliberate killing of civilians, the disproportionate nature of nuclear attack would still be clear: the victims of nuclear weapons include those killed or harmed by radiation, and the suffering caused by the damage to the land and to future generations. Such side effects are unacceptable even if unintended; since their inevitability is clear, it is unreasonable to conditionally accept them.

The authors of Nuclear Deterrence, Morality and Realism limited their factual claims; they were assessing only the intentions of the United States nuclear deterrent as it existed up until the late 1980s. Conditions have since changed, most obviously with the fall of the Soviet Union. But is it clear that the United States and other Western countries have renounced the nuclear deterrent? Far from it. And any such suggestion is surely belied by the worry that the present war in Ukraine could herald a major nuclear conflict, a possibility that appears to be predicated on the likelihood of the West’s response to any Russian nuclear use with reprisals, and a subsequent escalation.

What actions are called for if one believes the nuclear deterrent is immoral? If deterrence breaches the principle of discrimination, the only moral response is immediate disarmament, for that is the clearest way to renounce the threat of nuclear reprisals. And so it is not a great leap from the conclusions of Finnis, Boyle, and Grisez to that of Pope Francis, that it is impermissible not just to threaten the use of nuclear weapons but even to possess them.

The pope did not affirm the specific argument of Nuclear Deterrence, Morality and Realism; popes rarely wade into the arguments of lay thinkers. Moreover, Francis’s own stated reasons to this point might strike one as somewhat more prudential, emphasizing as he does “the risk of an accidental detonation as a result of error of any kind.” There is also the possibility that tactical nuclear weapons might figure into a morally permissible deterrent in a way that strategic weapons do not. So there perhaps remains a lacuna to be filled between the always and everywhere impermissibility of deliberately killing the innocent – Anscombe’s position – and the pope’s ending point.

Nevertheless, the current state of Catholic and just war thinking may be captured in three principles. First, there remain circumstances in which the use of military force is both permissible and requisite, primarily for the defense of the innocent. Second, there is a need for responsible statesmen to communicate some kind of deterrent threat against hostile states, namely, that if war is undertaken against a non-aggressing nation, steps will be taken to restore peace and order, not necessarily limited to economic sanctions. These two principles both follow from the tasks of political authority, foremost among them protection of the governed. And the two principles can be extended: the bonds of a common humanity can also call for the threat, or even the deployment, of a sovereign state’s military to protect the innocent of another nation from, for example, that nation’s gross violation of its citizens’ human rights.

But third, the maintenance of a nuclear arsenal that sits behind these more conventional norms and responsibilities is in itself a violation of the norms of war and the responsibilities of upright statecraft because it involves a conditional intent to kill innocents. That third principle sits awkwardly next to the first two. It seems that its inevitable consequence is quietism, a giving-in to the bully with the nuclear weapons. For how are protection of the innocent or permissible forms of deterrence against nuclear powers possible if the ultimate deterrent must be unilaterally renounced?