Since Russia invaded Ukraine in February 2022, the debate on Catholic teaching on peace has taken a turn. As Tobias Winright recently wrote in Commonweal: “This war tests…the recent narrowing of the Catholic ethic of war and peace to nonviolence.” Indeed, thinking about Ukraine’s response to Russian aggression in the context of Catholic moral tradition requires a re-reading of Pacem in Terris—the highest moment in the modern magisterium of the Catholic Church on peace and war.

The encyclical was dated April 11, 1963—Holy Thursday—but it had been signed on April 9, in an event that was televised around the world. Images of Pope John XXIII signing his last encyclical (he would die less than two months later) filled the news. The New York Times published the text in its entirety in its April 11 edition. Pravda, the official newspaper of the Communist Party of the Soviet Union, professed Pacem in Terris “an initiative in favor of peace.” Mainstream and other publications all across the globe devoted similar attention.

Pacem in Terris arrived six months after the Cuban missile crisis, and largely because of it. The most dangerous moment in the history of the Cold War compelled John XXIII to ask Italian theologian Fr. Pietro Pavan to draft an encyclical on peace. The document and the attention it received reflected the central role the Catholic Church played in articulating the moral language of the nuclear age—a language that developed previous doctrine in some important ways.

Sixty years later, the war in Ukraine—along with the marginal and somewhat controversial role of Pope Francis and the Vatican—affects our reading of Pacem in Terris. Neoconservatives are using the occasion to highlight the encyclical’s “deficiencies.” That’s an ahistorical and disingenuous assessment. Nevertheless, certain pressing developments do force us to reconsider it in a new context. For one thing, the abandonment of nuclear-weapon agreements could herald a new arms race and raises the risk of nuclear proliferation. As the Holy See delegation to the UN stated in April during the General Exchange of Views at the United Nations Disarmament Commission: “Tragically, the lessons of the Cuban Missile Crisis are being forgotten and numerous arms control treaties have been discarded, reflecting a paucity of trust internationally and accelerating a worrying trend toward rearmament.” Additionally, the plight of Ukraine—a country that gave up its nuclear weapons in 1994 and is now defending itself against a nuclear power—might inspire other non-nuclear countries to seek out or develop their own nuclear weapons in hopes of deterring invasion. Finland’s April decision to join NATO, meanwhile, deprives Europe of a country whose neutrality was critical in formulating the 1975 Helsinki Accords—hammered out at a peace conference it hosted, and to which the Holy See made significant contributions. And as the invasion leads other countries to reconsider their longstanding neutrality, the Holy See’s recent invocations of a “new Helsinki” seem more and more like a dream.

There are less visible considerations, but they are no less profound in terms of the history of theology. In addressing the issue of peace and war, Pacem in Terris also founded a new Catholic language of rights—human, social, political, and economic—in ways that were foundational for the documents of Vatican II, including the one on religious liberty, Dignitatis Humanae. Pacem in Terris also opened the door for social and political cooperation between Catholics and non-Catholics, thanks to its distinction between “error and the errant” as a response to the political ideologies and mass political parties of the Cold War. This distinction has become much more difficult to maintain and less useful in de-escalating tensions as identity politics and the theologization of the “culture wars” assumes an identification between “error and the errant.” If the Church was successful in using the distinction to deal with ideological clashes, it is finding it harder to apply when the emphasis is on identity.

There are also considerations in terms of the history of war. Pacem in Terris was crafted in the aftermath of the Cuban missile crisis but was also informed by the lived experience of World War II—“total warfare” and America’s use of atomic weapons against Japanese civilians in August 1945. But since the encyclical, we have seen the spread of what are euphemistically called “police operations” and “humanitarian wars.” We have seen the use of remotely controlled weapons that distance the attacker from the attacked and create the false impression of “surgical strikes” that hit their targets with precision while sparing civilians. Meanwhile, weapons that once could only be developed by a wealthy state power can now be made with technology found in consumer electronics; one of the most successful drones used by Russia against Ukraine is the low-tech Iranian Shahed-136, which costs $20,000 dollars.

“Shahed” in Persian means “witness,” as in “I bear witness that there is no God but God.” In 1963, religious rhetoric in support of war was not as common as it is now, especially in Europe. The heated debate over collaboration with Moscow against the non-autocephalous Orthodox Church in Ukraine, and the justification for Putin’s aggression provided by the Patriarch of Moscow, are just two examples of how ethno-nationalist leaders and public figures around the world—from the United States to Russia, India, Hungary, and Italy—are making a rhetorical connection between religion and war. It’s a problem for Christianity, Hinduism, and even Buddhism: states with substantial Buddhist populations have armies today.

Pacem in Terris’s famous condemnation of nuclear war as “irrational”—in the powerful Latin translation of the original Italian, alienum a ratione—and the recourse to “human reason” sit uncomfortably alongside the return of anti-Enlightenment irrationalism (visible in the mainstreaming of conspiracy theories) and, in Christianity, the re-emergence of the apocalyptic-driven alliance between religion and politics. In the case of the Moscow patriarch’s justification of the war in Ukraine, the concept of a “metaphysical” struggle against a godless international order is a complete contradiction of the rational approach of Pacem in Terris and a sign of the collapse of the post–Vatican II ecumenical dialogue (and not just between the Vatican and Moscow). Pacem in Terris invested in the rationality and the scientific reachability of peace based on a sense of “progress” (a word that appears twenty-one times in the encyclical), which since then has lost much of its power to persuade and appease. Pope Francis, who has talked many times about “the third world war being fought piecemeal,” has adopted a more humble and plainly less progressive-scientific language—that of being “artisans of peace.”

The most dramatic change affecting Pacem in Terris today is the representative nature of papal teaching on peace and war in global Catholicism. It has become harder to believe in not just the unity of the one human family, but also in the unity of the one Catholic Church. The difficulty Francis has had in taking a position on the war in Ukraine is attributable in part to his lamentable lack of verbal discipline in the too many interviews he has given lately. (It will be interesting to see what comes of Francis’s meeting with Ukrainian President Volodymyr Zelensky on May 13.) But it also reflects a real lack of consensus in the global Catholic community—and the gap between Catholics in the Anglo-American West and Eastern Europe who support Ukraine versus those in the Global South who see Putin’s invasion as a European war and support for the invaded country as support for the United States and its interests. Francis himself hews to interpretations of the nonaligned world more than “the free world” in ways that we cannot find in his predecessors since John XXIII.

After the publication of Pacem in Terris, President John F. Kennedy requested an audience with John XXIII, who died before it could take place, on June 3, 1963; Kennedy himself would be assassinated a few months later. Reading Pacem in Terris today, when the United States has its second Catholic president, brings into relief how politically impotent Catholics and the papacy have become since that time. The post-conciliar consensus arose from the fact that Pacem in Terris reflected an understanding of the world transitioning from an imperial-colonial order to the Cold War of ideological empires—the so-called “free world” against the atheistic Communist totalitarianism. That consensus is now in crisis, and the inversion of roles is complete: a “God-with-us” Russian regime against an anathematized, decadent, secular West.

This civilizational narrative is clearly ideological and instrumental. But sixty years on we can say that Pacem in Terris has been vindicated, especially in its insistence that “human coexistence must be considered above all as a spiritual fact.”