Pope Leo XIV on Monday (May 25, 2026) labelled “outdated” the concept of a “just war” put forward by U.S. President Donald Trump’s administration, warning that “humanity is slipping into a violent culture of power”.

In his first encyclical, a sort of manifesto, U.S.-born Leo focused mainly on the rise of artificial intelligence but also returned to an anti-war message that has caused tensions with Washington.

“Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the ‘just war’ theory, which has all too often been used to justify any kind of war, is now outdated,” he wrote in Magnifica Humanitas (Magnificent Humanity).

“Humanity possesses far more effective and capable tools for promoting human life and resolving conflicts, such as dialogue, diplomacy and forgiveness,” he wrote.

The Pope has been a high-profile critic of the West Asia war which started with U.S. and Israeli air strikes on Iran in February.

He condemned Trump’s threat to destroy Iran as “truly unacceptable” and urged Americans to demand that U.S. lawmakers “work for peace”.

Mr. Trump slammed the pontiff as “WEAK on Crime, and terrible for Foreign Policy” — to which Pope Leo replied that he had a “moral duty to speak out”.

The Trump administration has repeatedly justified its attack on Iran as a way to prevent the country from developing a nuclear weapons capability.

U.S. Vice President J.D. Vance, a Catholic convert, has used the “just war” argument and has urged the pope to be “careful” on theological matters.

In an interview earlier this month, Mr. Trump again criticised the pope, alleging that Leo believes it is “OK for Iran to have a nuclear weapon”.

“I think he’s endangering a lot of Catholics and a lot of people,” Mr. Trump said.

Asked about the comments, Pope Leo said that the Catholic Church’s mission was to “preach peace” and the Gospel.

“If anyone wishes to criticise me for proclaiming the Gospel, let them do so truthfully,” he told reporters.

U.S. Secretary of State Marco Rubio met with the Pope earlier this month in an attempt to repair relations.

 U.S. Secretary of State Marco Rubio arrived at the Vatican on Thursday (May 7, 2026) for talks with U.S.-born Pope Leo XIV, following weeks of tensions between the Vatican and the White House.

His fence-mending visit comes after President Donald Trump’s broadsides against Pope Leo and the U.S.-Israeli war in Iran angered the Holy See and sparked ongoing sparring between the two American leaders.

Mr. Rubio, a practising Catholic, had an audience scheduled with Pope Leo, which was complicated at the last minute by Mr. Trump’s latest criticism of the Chicago-born pope.

Pope Leo has pushed back, calling out Mr. Trump’s misrepresentations of his views on Iran and nuclear weapons and insisting that he is merely preaching the biblical message of peace.

Mr. Rubio was also due to meet with the Vatican secretary of state, Cardinal Pietro Parolin, who on the eve of his visit strongly defended Leo and criticised Mr. Trump’s attacks in understated diplomatic terms.

“Attacking him like that or criticising what he does seems a bit strange to me, to say the least,” Mr. Parolin said on Wednesday (May 6, 2026).

Meetings on Friday (May 8, 2026) with Premier Giorgia Meloni and Foreign Minister Antonio Tajani might not be much easier for America’s top diplomat, given both have strongly defended Pope Leo against Mr. Trump’s attacks and have criticised the Iran war as illegal — drawing the president’s ire.

Rubio insisted this week that the visit had been in the works for a while but that “obviously we had some stuff that happened”.

Vatican seen as willing to have dialogue

Giampiero Gramaglia, former head of the ANSA news agency and its onetime Washington correspondent, said he didn’t expect much to come out of Mr. Rubio’s visit for Italian or Vatican relations.

He, and other Italian commentators, believe Mr. Rubio instead was looking to smooth over relations with the pope for his own political ambitions as well as the upcoming midterm congressional elections and 2028 presidential race.

“I doubt Rubio has the role of conciliator for Mr. Trump,” he told Italy’s Foreign Press Association. “I have the perception that Mr. Rubio’s mission is more about himself” and his political ambitions as a prominent Catholic Republican.

The Rev. Antonio Spadaro, undersecretary in the Vatican’s culture office, said Rubio’s mission wasn’t to “convert” the pope to Mr. Trump’s side. Rather, Washington “has come to acknowledge — implicitly but legibly — that (Leo’s) voice carries weight in the world that cannot simply be dismissed.”

“The situation created by President Trump’s remarks required a high-level, direct intervention, conducted in the proper language of diplomacy: a semantic corrective to a narrative of frontal conflict with the church,” he wrote in an essay this week.

Journalist Massimo Franco, writing in the Corriere della Sera newspaper, said the Vatican’s decision to not cancel the pope’s audience with Mr. Rubio after Mr. Trump’s latest broadside was evidence of its willingness to keep open dialogue.

But relations with the Meloni government, which is facing widespread Italian public opposition to the Iran war, are not so easily smoothed over. “Keeping the alliance with the United States firm while criticizing the president is showing itself to be increasingly difficult,” Mr. Franco wrote.

Cuba is also on the agenda

Mr. Rubio said topics other than the Iran war were on the agenda for the Vatican visit, including Cuba. The Holy See is particularly concerned about the Trump administration’s threats of potential military action there following its January ouster of Venezuelan President Nicolás Maduro.

Mr. Trump has said frequently that Cuba could be “next” and even suggested that once the war with Iran is over, naval assets deployed in West Asia could return to the United States by way of Cuba.

Mr. Rubio is the son of Cuban immigrants and a longtime Cuba hawk.