When US Vice President JD Vance converted to Catholicism in 2019, he chose Saint Augustine as his patron.

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On Tuesday, speaking at a Turning Point USA event, Vance invoked the tradition of the fifth-century theologian and one of the most important Church fathers to push back against Pope Leo XIV’s criticism of the war in Iran.

The White House number two warned the pontiff to “be careful when he talks about matters of theology,” citing “more than a 1,000-year tradition of just war theory” in his defence.

Meanwhile, the supreme pontiff of the Catholic Church was in the Algerian port city of Annaba, paying homage at the basilica not far from where St Augustine died and was initially interred.

Hippo Regius, as it was known in the bishop’s time, is where St Augustine wrote most of what became the intellectual basis of the just war principles Vance was claiming to defend. Pope Leo XIV is the first pontiff to hail from the Augustinian order.

Whether Vance knew what the Holy Father’s itinerary was that day, his office did not say.

Vance was not the first member of the administration to weigh in.

Days earlier, US President Donald Trump had posted on Truth Social and later reiterated to the press that Pope Leo XIV was “weak on crime” and “terrible for foreign policy,” suggesting the pontiff believed Tehran should be allowed to develop nuclear weapons.

The pope never made any comments regarding the Islamic Republic’s right to nukes.

The post came after the pope had called Trump’s threat to destroy Iran’s “whole civilisation” “truly unacceptable”.

Pope Leo XIV responded the following morning on board the papal plane to Algiers. “I’m not afraid of the Trump administration or of speaking out loudly about the message of the Gospel,” he said.

“I will continue to speak out loudly against war, looking to promote peace, promoting dialogue and multilateral relationships among the states to look for just solutions to problems.”

What the doctrine says

Just war theory, rooted in St Augustine and further elaborated on by Thomas Aquinas in his Summa Theologiae, sets out strict conditions for the moral use of military force.

The threat must be lasting, grave and certain, and success must be realistically achievable. Most importantly, all other means of resolution must be genuinely exhausted, and the harm caused must not exceed the harm it seeks to prevent.

Put simply, the purpose of this set of rules is to prevent those engaged in war from being the final judges of their own righteousness.

“The just war doctrine doesn’t merely ask whether your cause feels just,” Joseph Capizzi, Dean of the School of Theology and Religious Studies at the Catholic University of America, told Euronews. “As we all know, everybody thinks their situation is just.”

“It understands that most people think of their causes as just. But it is a means by which you can distinguish legitimately just causes of war from illegitimate causes of war.”

The doctrine has also shifted in how it is applied. For most of its history, it was used by priests to authorise their rulers’ wars. Spurred on by world wars and the discovery of nuclear weapons, the modern papacy has used it in the other direction.

“Before, just war doctrine was used often by national clergy to give permission to their emperor or their king to go to war,” Massimo Faggioli, professor of ecclesiology at Trinity College Dublin told Euronews.

“Right now, it is used mostly — I would say almost always — to say ‘well, no, this military intervention doesn’t meet those criteria.’”

Writing as the Roman Empire crumbled, St Augustine had already posed the question of what is righteous in one of the most well-known open checks on power in Catholic moral thought.

“Justice removed,” he asked in The City of God, “what are kingdoms but great bands of robbers?”

Vance has cited The City of God as “the best criticism of our modern age” he has ever read, deeply affecting his religious outlook and thoughts on domestic and foreign policy.

Vatican’s track record

The administration’s framing of Pope Leo XIV as a pacifist who simply does not understand that force is sometimes necessary contradicts the pontiff’s and the Church’s track record, experts say.

Before his election just last year, the pontiff was a registered Republican voter. While he has criticised the Iran war, the Holy Father has shown support for Ukraine’s right to self-defence.

In recent decades, past popes also carefully deliberated the context before commenting on any given conflict.

The Holy See quietly regarded the post-September 11 intervention in Afghanistan as meeting just war criteria, as the US went after Taliban extremists and Al-Qaeda’s Osama bin Laden.

Yet Pope John Paul II opposed both the 1991 Gulf War and 2003 invasion of Iraq not as a pacifist, but on the grounds that last resort had not been demonstrated. Pope Leo XIV’s position on Iran is in line with his predecessors, according to theologians.

“To accuse the pope of being a pacifist is really absurd,” Faggioli said. “Vance and Trump are accusing the pope of thinking about war like a European Catholic. But that’s not true.”

“He is using just war doctrine — and the American cardinals who have spoken against the war in Iran, they have used just war doctrine in ways that Europeans would not. So this is, in some sense, an intra-American debate.”

There is also the matter of what Vance actually said — not just about just war, but about the pope’s remit, after he suggested Pope Leo XIV should confine himself to morality and stay out of foreign policy, Faggioli explained.

“Vance is one of those typical Catholics who thinks that morality is only sexual morality,” Faggioli said. “When he said the pope should stick only to morality, he meant sexual morality — as if war were not a matter of morality. Of course it is.”

Thousand-year tradition and its tenets

The US bishops and other Catholic Church clergy indeed did not stay quiet. On Wednesday, Chairman of the USCCB Committee on Doctrine Bishop James Massa issued a statement in support of the Holy Father’s position, but also the Catholic Church as a whole.

“A constant tenet of that thousand-year tradition is a nation can only legitimately take up the sword ‘in self-defence, once all peace efforts have failed,’” Massa, auxiliary bishop of Brooklyn, wrote.

“When Pope Leo XIV speaks as supreme pastor of the universal Church, he is not merely offering opinions on theology. He is preaching the Gospel and exercising his ministry as the Vicar of Christ.”

Unlike in other public exchanges in recent times with those opposing Washington’s view, the Trump administration has struggled to find the usual levers, experts say. “It’s very hard for them to use the usual tactics to delegitimise the pope, because he is American,” Faggioli said.

“They can’t call him a communist, they can’t call him a radical leftist — his record as a theologian doesn’t support that.”

Euronews contacted several Catholic institutions and theologians for perspectives to further outline the Trump administration’s application of just war doctrine, but none agreed to speak on the record.

‘A consistent lesson of our faith’

On Thursday, from a peace meeting in Cameroon — a country not without its own existing tensions — the pope said, “Woe to those who manipulate religion and the very name of God for their own military, economic, and political gain, dragging that which is sacred into darkness and filth.”

The post on X from his official Pontifex account drew nearly 10 million views in English alone by Friday evening.

Capizzi urged against reading every papal statement as aimed at Washington, however. “You’re in Cameroon, on a continent marked by severe religious conflict; that comment has a much broader application.”

Still, according to Capizzi, the Holy Father’s words are meant for all of the faithful.

“Any believer who appeals to God — as though God is on their side — ought to do so with great fear and trembling,” he said. “That is a consistent lesson of our faith: that a believer is the person who has a healthy fear of God and of God’s judgment of his or her actions. And that includes the way he or she speaks about God.”

The same day at the Pentagon, US Secretary of Defence Pete Hegseth led a worship service and read what he described as a prayer recited by Combat Search and Rescue crews during the Iran operation.

He introduced it as “CSAR 25:17,” meant to reflect Ezekiel 25:17. What followed was nearly verbatim the monologue delivered by Samuel L Jackson’s hitman in Quentin Tarantino’s Pulp Fiction, in the scene immediately before his character Jules Winnfield commits a murder.

The actual Ezekiel 25:17 is considerably shorter and less specific. Tarantino’s version was itself adapted from a 1973 Japanese martial arts film.

‘Nothing against the pope’

Trump won around 55% of US Catholic votes in 2024. A poll conducted in late March, jointly by Republican pollster Shaw & Co Research and Democratic pollster Beacon Research, found his approval among Catholics had fallen to 48%, with 52% disapproving.

A Fox News poll found US Catholics opposed to military action in Iran by 10 points and against Trump’s conduct toward Iran by 20. A separate NBC survey found US registered voters now view the pope more favourably than the president by a net margin of 46 points.

On Thursday, Trump told reporters he has “nothing against the pope” and is “all about the Gospel,” while continuing to state Pope Leo XIV was in favour of Tehran having nuclear weapons.

Trump also said his preference remained with the pope’s brother Louis, who lives in Florida. “Louis is all MAGA. He gets it, and Leo (XIV) doesn’t,” Trump said.

“If I wasn’t in the White House, Leo wouldn’t be in the Vatican,” he reiterated.

The night before, police had surrounded the New Lenox home of a different brother of the pope, John Prevost, following a bomb threat. K9 explosive-detection units found nothing. The investigation remains ongoing.

The greater picture

For Faggioli, the dispute is a symptom of something that has been building for years: not a domestic row about one war, but a contest over what Christianity means and who speaks for it.

“America always had a religious understanding of itself as a nation, but presidents were very cautious about not looking like messianic figures — at least in life,” Faggioli said.

“Trump has exploited the creation of a vacuum of secularisation in America, and he has filled that vacuum with a certain degree of messianism — and some American Christians are happy about that.”

“Trumpism is a form of political messianism. He sees himself — and many people see in him — someone with a divine mission: a political Messiah who will deliver salvation to America, to Americans, to Christianity. And he is serious when he posts those things.”

Capizzi, for his part, was more of the belief that the US president would eventually mend bridges with the Holy See. “I actually consider this a hopeful sign — that it’s touching and impacting President Trump, despite what he’s saying and what he’s posted.”

“This conversation has shown that the Church retains her moral authority,” he said.

“This is a teaching moment. Catholics and others are getting to see that these doctrines are over a thousand years old, that we have thought about these questions for a very long time, and there is a moral gravity behind these claims.”

As for the pope, John Prevost said something crucial about his brother before any of this began. “I don’t think he’ll stay quiet for too long if he has something to say,” he told the New York Times last year. “He won’t just sit back.”