The Power that Preserves the Peace

In a world at war, and with wars rarely being a real solution, the nuclear arms stalemate has been a realistic compromise between Catholicism’s pacificism and its just war theory.

Author: Michael C. Desch

Banner Desch Illustration by David Pohl

In 2020 I chose the following lines from the Book of Isaiah for the Notre Dame International Security Center’s Christmas card:

In days to come,

The mountain of the LORD’s house

shall be established as the highest mountain

and raised above the hills.

All nations shall stream toward it.

One nation shall not raise the sword against another,

nor shall they train for war again.

This might seem like an odd scriptural selection. I am, after all, the director of a research center devoted to studying the role of military force in international relations and preparing undergraduate and graduate students — civilian and military — for careers in national security. These future soldiers and policymakers may soon be raising the sword against another nation or supporting those who do. Of course, the current Ukraine crisis, pitting the United States and its NATO allies against Russia, is only the most recent illustration that people like Vladimir Putin may believe that war is still the answer in great power politics. Am I admitting that war is never the answer and that my students are wasting their careers and imperiling their very souls?

By no means. While all Christians should keep their eyes focused upon the “days to come” when we can beat swords into plowshares, we are not there yet. Moreover, to the extent that we are slowly getting to a world in which major war among great powers is becoming less relevant, it is not due to Gospel pacifism but rather is a result of the nuclear revolution.

The “absolute weapon,” as political scientist Bernard Brodie called nuclear armaments in a 1946 book, marked a revolution in military affairs by foreclosing on the use of military force as a viable instrument of statecraft among major powers. Brodie famously explained that “thus far the chief purpose of our military establishment has been to win wars,” but “from now on its chief purpose must be to avert them. It can have almost no other useful purpose.” And the purpose of the threat of “mutual assured destruction” is to deter war and keep the peace, at least of a sort.

Does the Russian invasion of Ukraine cast doubt upon the nuclear revolution? Absolutely not. The nuclear revolution changes the calculus of conflict between two nuclear powers; it does not promise to end all military conflict. Indeed, the Ukraine war makes this point in two ways: First, a growing number of analysts are belatedly coming to realize that Ukraine’s decision to give up its Soviet nuclear inheritance in the mid-1990s may have been a mistake. If Ukraine had remained a nuclear power, there likely would be no Russian tanks on the streets of Kyiv today. Second, the Biden administration has been very careful to limit its support of Ukraine to economic sanctions and indirect military aid. As the president noted on the eve of the war, “Let me be clear . . . we have no intention of fighting Russia.” At the heart of his reticence is his recognition that a war between the U.S. and Russia could risk nuclear Armageddon. 

Much of the perceived tension between arming for war and desiring peace stems from the Christian pacifist argument that war and peace are polar opposites. Such a dichotomous view is emblematic of the position of the modern Church, especially following the Second Vatican Council, but it is at variance with the Church’s long tradition of just war theory, in which military force, under certain limited circumstances, could achieve and maintain peace.

As the bishops argued in their 1983 pastoral letter, The Challenge of Peace, “the Catholic tradition on war and peace is a long and complex one” that oscillates between the poles of “nonviolence” and “just war.” At present, we are living under the ascendency of what Pope St. John Paul II’s biographer George Weigel calls “just war forgetfulness,” by which he means that the pendulum of Catholic social teaching has now swung decisively toward nonviolence.


We would be better served, in my opinion, by returning to the just war tradition and embracing Christian realism. To put the Christian realist position in context, it is necessary to understand the evolution of the Church’s thinking on the use of force. The thrust in early Christianity was toward pacifism and nonviolence. As the Lord said in the Sermon on the Mount, “Blessed are the peacemakers, for they will be called children of God.” During his passion, he rebuked the disciple who tried to defend him in the Garden of Gethsemane from the high priests and their temple guards: “Put your sword back into its sheath; for all those who take the sword will perish by the sword.” It is on this basis that many modern Catholics regard pacifism as the core of Jesus’ teaching.

It is important to keep two things in mind about this period. First, most commentators in the early Church interpreted the Lord’s call to repentance as evidence of his imminent return and the end of days. Given that expectation, temporal affairs undoubtedly seemed irrelevant. Second, for its first three centuries, the Church constituted a persecuted minority whose members were kept away from the levers of political power. This meant it was largely spared the difficult practical and moral challenges of reconciling the needs of temporal authority with the strictures of Christ’s Gospel.

All of this changed early in the fourth century, when the Roman Emperor Constantine converted to Christianity. The previously marginal and persecuted Christian cult became the official faith of an enormous empire. Moreover, after more than 300 years of waiting for the Lord’s return, it dawned on the faithful that their residence in the earthly city might be long-term. Just war theory arose from the need for devout Christians to be able to effectively conduct temporal affairs for an indefinite period of time in a world where the use of military force was often the answer to political and even moral problems.

One can certainly find Gospel foreshadowing of just war theory, especially the Lord’s injunction to Roman soldiers not to beat their spears into pruning hooks but rather not to extort or “falsely accuse” anyone and to “be satisfied” with their wages. But St. Augustine was the true wellspring of the Catholic just war tradition. Central was his distinction in The City of God between the heavenly city and the earthly city, the latter dominated by the “lust of rule” rooted in our flawed humanity. Given our proclivities, the bishop of Hippo reserved “perfect peace” exclusively for heaven. “In the great ebb and flow of human affairs,” he explained, “no people has ever been granted such security that it did not have to fear attacks on its life here below. That place that has been promised for that kind of peace and security is the eternal one.” He told Count Boniface, the Roman general and governor of northern Africa, that believers may have to “go to war that we may have peace.”

Catholic just war thinking developed further under the influence of St. Thomas Aquinas and was eventually codified both in terms of when it is licit to go to war — jus ad bellum — and how to conduct war justly: jus in bello. To be considered licit, a war required a just cause, such as the defense of innocents. The state needed to retain the moral high ground and to represent legitimate authority as a polity animated by Christian principles. Further, such a war required the right intention (tough love of one’s neighbor, or what Augustine called “benign severity”) and a reasonable chance of success, and it had to be pursued in a manner proportional to the provocation. Finally, war had to be the last resort. Diplomacy and all other instruments of statecraft needed first to be exhausted. Principles guiding the just conduct of war mandated that it be fought consistent with these criteria.


Exactly when we reached the end of the “Constantinian epoch,” in Weigel’s words, is hard to pinpoint. Was it with the fall of the Roman Empire? The Reformation? Or the 1929 Lateran Treaty, in which Italy formally reduced the Holy See’s temporal sovereignty to the mere 109 acres we call Vatican City? Regardless, the Second Vatican Council put an exclamation point on this diminishment. By then, the Church had long since lost any temporal power in the international system — a point the Soviet dictator Josef Stalin dismissively underscored (“The Pope! How many divisions has he got?”) — and the Reformation’s legacy of the separation of Church and state had undermined its residual moral influence upon the conduct of statecraft.

Vatican II revolutionized many elements of the Church and of Catholic thinking. Consideration of the justness of war was no exception, the notion itself being replaced with a “presumption against the use of force.” But the proposed revolution in Catholic thought went well beyond the mere avoidance of war. Pope John XXIII’s 1963 encyclical, Pacem in Terris, calls for a total transformation of human society:

Everyone, however, must realize that, unless this process of disarmament be thoroughgoing and complete, and reach men’s very souls, it is impossible to stop the arms race, or to reduce armaments, or — and this is the main thing — ultimately to abolish them entirely. Everyone must sincerely cooperate in the effort to banish fear and the anxious expectation of war from men’s minds. But this requires that the fundamental principles upon which peace is based in today’s world be replaced by an altogether different one, namely, the realization that true and lasting peace among nations cannot consist in the possession of an equal supply of armaments but only in mutual trust. And we are confident that this can be achieved, for it is a thing which not only is dictated by common sense, but is in itself most desirable and most fruitful of good.

The peace the pope had in mind “does not consist merely in the absence of war, but rather in sharing the goodness of life together.” To achieve it would require a change of global priorities to focus on the common good instead of national interests. Such a peace would promote social and economic development and establish solidarity between rich and poor nations. Perhaps the Holy Father’s most radical break with the just war tradition was the implication that the true peace of the heavenly city could be approximated in the here and now.

The core of Francis’ rejection of just war is that ‘every war leaves our world worse than it was before. War is a failure of politics and of humanity, a shameful capitulation, a stinging defeat before the forces of evil.’ We must grasp, he says, the ‘abyss of evil at the heart of war.’

John XXIII, in the political scientist and U.S. foreign policy adviser Hans Morgenthau’s view, had thereby proclaimed “peace as the paramount interest of the Church and of mankind.” In the mind of the Church, peace was no longer merely “preferable to war on moral grounds,” as it has been in the tradition of the Church and Western civilization, Morgenthau observed. “Nor does the pope distinguish, as his predecessors did, between just and unjust wars. War now has become an evil per se, and in consequence all wars have become equally unjust. Both reason and morality require the preservation of peace. The Church Militant has become the Church Pacificant.”

Subsequent popes may have emphasized different notes in this tune, but they have consistently sung from the same pacifist sheet music. Even John Paul II, acknowledging the relative peace of the Cold War, still declares in Centesimus Annus that “true peace is never simply the result of military victory, but rather implies both the removal of the causes of war and genuine reconciliation between peoples.”

Our current pontiff, Pope Francis, is even more categorical about the moral imperative to supersede the just war tradition. His encyclical Fratelli Tutti insists that we “can no longer think of war as a solution, because its risks will probably always be greater than its supposed benefits. In view of this, it is very difficult nowadays to invoke the rational criteria elaborated in earlier centuries to speak of the possibility of a ‘just war.’ Never again war!”

The core of Francis’ rejection of just war is that “every war leaves our world worse than it was before. War is a failure of politics and of humanity, a shameful capitulation, a stinging defeat before the forces of evil.” We must grasp, he says, the “abyss of evil at the heart of war.”

The Church now largely rejects any use of military force as incompatible with Gospel teachings. Under Francis, it has gone further and pronounced even deterrence — the discouragement of hostilities through the threat of the use of force — as anathema, particularly deterrence of the nuclear variety. This perspective increasingly holds that the use, or even the threat, of military force is no longer the answer to political and moral conflict between nations, and likely never was.


Christian realism cautions, in contrast to the Church’s view, that dismissing the just war tradition as a wrong turn on the road from the earthly to the heavenly city is an error. While the utility of military force as an instrument of statecraft, particularly among the great powers, has surely declined — witness what the Yale historian John Lewis Gaddis has called the unexpected “long peace” of the Cold War — it would be a mistake to ignore the role that the nuclear revolution played in this change. To be sure, nuclear deterrence poses challenging practical problems for political leaders and raises grave theological issues. But it is better than the naive pacifism that leads many Catholics to reject the peace it offers in favor of a utopian vision of a world without war and nuclear weapons.

Articulated by the 20th century Protestant theologian Reinhold Niebuhr, Christian realism is an ecumenical perspective that finds its roots in the Roman Catholic just war tradition. Christian realists recognize that military force remains central to modern international relations. After the nuclear revolution, one facet of this force is the purposeful nonuse of nuclear weapons. Christian realists endorse in good faith Isaiah’s eschatologically pacifist sentiments but recognize that their relevance in the world remains limited.

Modern Christian realists harken back to St. Augustine in distinguishing between the imperfect earthly city and the perfect heavenly city. According to Niebuhr, Augustine’s “whole strategy for the ‘commingling’ of the two cities revolves around the acceptance of the ordinary responsibilities of home and state but in performing these tasks for the ultimate, rather than the immediate, end.” The challenge for the Christian realist is to seek salvation while living in a flawed world.

Post-Cold War realists have similarly embraced restraint to argue against recent wars like the nation-building effort in Afghanistan after 2001, the invasion of Iraq in 2003 and even the limited intervention in Syria after 2011.

In the nuclear age, war may no longer be the answer to unresolved conflict that it once was in such generally accepted cases of “good” — or at least “necessary” — wars as the American Revolution, the U.S. Civil War and World War II. Still, deterrence remains a central strategy for keeping the peace. Indeed, Gaddis’ “long peace” — the absence of a World War III in the second half of the bloody 20th century — is widely credited to the brooding shadow of nuclear annihilation.

For secular realists like Hans Morgenthau, the nuclear revolution simply ushered in a world of great-power peace. For Christian realists, recognizing the salutary impact of the nuclear revolution need not pose an insurmountable intellectual or moral challenge. An Augustinian orientation makes it possible to live with the paradox that, at least among nuclear-armed great powers, the obsolescence of war is a function of the nuclear revolution, mutual assured destruction and continuing nuclear deterrence.

The common denominator between the Catholic just war tradition and both schools of realism is restraint. During the Cold War, for instance, both Niebuhr and Morgenthau identified St. Augustine as one of the taproots of their arguments that flaws in human nature are an important reason why states need to take steps to ensure their survival and security. The same flaws also led them to be skeptical of conventional wars of choice like Vietnam, in which the U.S. has sought to remake countries in the developing world in its own image.

Post-Cold War realists have similarly embraced restraint to argue against recent wars like the nation-building effort in Afghanistan after 2001, the invasion of Iraq in 2003 and even the limited intervention in Syria after 2011. Even absent the nuclear revolution, realists increasingly believe that “humility” — a word that candidate George W. Bush campaigned on in 1999, but which President Bush forgot after 9/11 — counsels that war is rarely if ever the answer to our foreign problems, even when the U.S. is militarily preponderant.


Unfolding events pose practical and moral challenges to Christian realists and Christian pacifists alike. The latter maintain that continued reliance on nuclear deterrence is dangerous and costly. To be sure, the nuclear peace of the Cold War did not prevent expensive conventional wars, especially between nonnuclear powers. Yet despite humanity’s enduring fears of accidents and rogue nations, nuclear peace has proven durable and stable. And while nuclear weapons are expensive in absolute terms, compared to conventional armaments they are relatively cheap, constituting around 10 percent of current U.S. defense spending.

Christian pacifism faces two other serious challenges. The first is practical: Will it work, or is it based upon a simplistic view of international politics? Here, Pope Francis gives away the game when he states in Fratelli Tutti that it will not trouble him “to be deemed naive for choosing peace.” His solution is to ignore the problem. But the world remains recalcitrant. Despite more than a half-century of consistent anti-nuclear sentiment from the Church, we still live in a world of nuclear weapons.

The second difficulty is logical: Vatican II acknowledged, but did not fully wrestle with, the fact that it was the most horrific of modern weapons that made credible the notion “that war is no longer the answer.” As John XXIII confessed, “We acknowledge that this conviction owes its origin chiefly to the terrifying destructive force of modern weapons. It arises from fear of the ghastly and catastrophic consequences of their use. Thus, in this age which boasts of its atomic power, it no longer makes sense to maintain that war is a fit instrument with which to repair the violation of justice.”

The council further admitted in Gaudium et Spes that “scientific weapons are not amassed solely for use in war. Since the defensive strength of any nation is considered to be dependent upon its capacity for immediate retaliation, this accumulation of arms, which increases each year, likewise serves, in a way heretofore unknown, as deterrent to possible enemy attack. Many regard this procedure as the most effective way by which peace of a sort can be maintained at the present time.” Ironically, the nuclear revolution helped set the stage for the Vatican’s embrace of radical pacifism just as successive popes were declaring nuclear deterrence morally unacceptable.

Christian pacifists in turn raise an especially valid moral objection for Christian realism. As the American bishops and many theologians have noted, mutual assured destruction’s retaliatory threat, if it is not a bluff, violates the just war tradition’s injunction against attacking noncombatants. But the French bishops dismissed this concern by noting, rightly, that threats are not use.

This response is compelling as far as it goes, but Christian realists should go further and embrace “existential deterrence.” While mutual assured destruction depends upon an explicit deterrent threat to inflict unacceptable damage on civilian targets — famously quantified by former Defense Secretary Robert McNamara as the destruction of one third of an adversary’s population and half of its industrial base — existential deterrence relies upon a small nuclear capability and the risk that any conventional great-power war might spin out of control. (This has long been China’s nuclear strategy.) Existential deterrence is also a better option than deterrence based upon counterforce — exclusively targeting the other side’s weapons and military forces — because in practice it would be very hard, if not impossible, to limit a nuclear war to military targets given that they are often located close to population centers.

Finally, what might we Christian realists say to hawkish opponents who believe war is the best way to stamp out evil, which remains all too present in the world? War, we argue, is permissible only in limited and extreme cases, such as an intervention to prevent genocide. Such limits are necessary, as Catholics know, because the road to hell is often paved with good intentions. Many recent efforts to use military force to bring justice to the world — fighting communism in Vietnam, nation-building in Afghanistan and even many noble humanitarian interventions in other places — have ended up making things worse. True justice, like true peace, will only come in the heavenly city — another reason why realism and just war theory are compatible in their common commitment to military restraint.


Michael Desch is Brian and Jeannelle Brady Family Director of the Notre Dame International Security Center and Packey J. Dee Professor of International Relations at the University of Notre Dame.