For centuries, the Vatican has maintained a complicated, often controversial role in political life, a history that continues to shape modern debates as Pope Leo XIV weighs in on war, power and nuclear weapons.
While critics periodically argue that popes should “stay out of politics,” historians and theologians say such a claim runs counter to the Catholic Church’s own history. From confrontations with Roman emperors to Cold War diplomacy, the papacy has long asserted a moral authority that crosses international borders.
“The church is, in a way, its own polity,” said Grant Kaplan, an associate professor of theology at Saint Louis University. “It has political ramifications because it is a kind of society, but that doesn’t mean it functions like a political party or a nation‑state.”
Confronting Power in the Ancient and Medieval Worlds
The Vatican’s political engagement predates the modern concept of church‑state separation. In the fourth century, St. Ambrose, bishop of Milan, publicly rebuked the Roman emperor Theodosius I after a massacre in Thessalonica, barring him from receiving Communion until he performed public penance.
During the Middle Ages, popes regularly intervened in European politics, at times forcing kings and emperors to submit publicly to church authority. One of the most famous examples came during the Investiture Controversy, when Emperor Henry IV was made to stand barefoot in the snow at Canossa while seeking absolution from Pope Gregory VII.
The Vatican’s exercise of power was not without errors. Pope Innocent III was pope during the Fourth Crusade. This period of history is frequently cited as an example of how papal involvement in warfare could lead to catastrophe.
War, Doctrine and the Limits of Violence
Central to the church’s political engagement is the development of Just War Doctrine, a doctrine designed to limit, rather than justify, armed conflict. The theory sets strict conditions for when war may be considered morally permissible, including self‑defense, proportionality and the protection of civilians.
Kaplan said the doctrine is often misunderstood or selectively cited in modern political debates.
“Anybody who reads the just war criteria seriously ends up saying almost no modern war qualifies as just,” he said, noting that civilian casualties and the threat of disproportionate force dominate contemporary warfare.
The church’s opposition to crusading language and religiously framed nationalism has deep roots. Kaplan described such rhetoric as “strictly opposed” by Catholic doctrine and historically linked to disaster when church authorities aligned themselves too closely with secular military power.
Popes and Modern Global Politics
In the 20th century, papal diplomacy expanded as global warfare intensified. Pope Benedict XV unsuccessfully attempted to broker peace during World War I, while Pope Pius XII’s actions during World War II remain the subject of ongoing historical debate.
Pope John Paul II is widely credited with playing a key role in opposing communist rule in Eastern Europe, particularly in his native Poland.
Since the atomic bombings of Hiroshima and Nagasaki in 1945, opposition to nuclear weapons has become a consistent theme of papal teaching. According to a Vatican News editorial, successive popes — including Pius XII, John XXIII, Paul VI, John Paul II, Benedict XVI and Francis — warned that nuclear arms threaten both human dignity and the future of civilization.
Pope Leo XIV and the Nuclear Age
Pope Leo XIV has continued that trajectory, repeatedly calling for nuclear disarmament and dialogue over confrontation. In statements issued between 2025 and 2026, Leo warned that nuclear deterrence is rooted in fear rather than justice and urged world leaders to abandon the logic of mutual destruction.
In a message marking the expiration of the New START Treaty, the pope called for renewed commitment to arms control, warning that a new arms race would further destabilize global peace.
“These are not partisan claims,” Kaplan said. “They are entirely consistent with the magisterium of the church going back decades.”
Politics Without Parties
Despite its long history of political involvement, the Vatican generally avoids formal endorsements of candidates or parties. Bishops’ conferences, including the U.S. Conference of Catholic Bishops, have issued voting guidelines for decades, urging Catholics to apply moral principles rather than partisan loyalty.
Still, tensions persist, particularly in the United States, where Catholic institutions have at times faced political backlash. Kaplan noted that anti‑Catholic sentiment has deep roots in American history and can resurface when church leaders challenge prevailing political narratives.
At the same time, the Vatican’s global outlook increasingly looks beyond the United States. Africa now has more Catholics than North America and Europe combined, a demographic shift that shapes papal priorities.
“The relationship with America is important, but it’s not central,” Kaplan said. “The church has always thought in global terms.”