ROME – Upon arriving in Rome to take part in this weekend’s G20 meeting, United States President Joe Biden will pay a visit to Pope Francis.

They have met three times before, both in Rome and on American soil, but this will be the first time they share facetime since the U.S. elected its second Catholic president last November.

According to White House National Security Advisor Jake Sullivan, the meeting between the president and pope will have an “obvious personal dimension,” since they have met before, and “exchanged letters.”

“They will have a chance to just reflect, each of them, on their views of what’s happening in the world,” he said earlier this week. “On policy issues, of course, in the international realm, they’ll be talking about climate and migration and income inequality and other issues that are very top of mind for both of them.”

Though the meeting comes amidst some rifts with the U.S. bishops on pro-life and LGBT issues, the list of things the two have in common is larger than the one of issues on which they disagree.

However, often during the previous 30 meetings between popes and U.S. presidents the line between what the commander in chief can do or refuses to do despite papal pressing hasn’t always been clear.

In 27 years, John Paul II held more than a dozen meetings with U.S. presidents. One of the most famous — and most uncomfortable for an American leader — came in 2004 with George W. Bush. After a long series of behind-closed-door meetings with heads of State, the Polish pontiff read a statement in English strongly condemning the Iraq war.

“It is the evident desire of everyone that this situation now be normalized as quickly as possible with the active participation of the international community and, in particular, the United Nations organization, in order to ensure a speedy return of Iraq’s sovereignty, in conditions of security for all its people,” the pope said.

He also spoke about the “threat of international terrorism,” calling it a source of “constant concern,” that affected the relations between peoples “since the tragic date of 11 September 2001, which I have not hesitated to call ‘a dark day in the history of humanity.’”

But that dark day in history, in the mind of John Paul II, did not justify the war, and he was convinced Bush could have stopped it.

Unphased by the – expected – criticism, Bush chose to cheekily turn the other cheek, telling reporters: “For those of you who have ever met him, you know, I’m telling you the truth when I tell you, being in his presence is an awesome experience.”

It was the last time Pope John Paul II would receive an American president before his death in 2005.

For Wilson, Italy was worth more than the Vatican

The first man who had sat in the Oval Office to meet a pope was Ulysses S. Grant, who paid a visit to Leo XIII. However, it came in 1878, after he’d left office. The first sitting U.S. president to meet a pope -Benedict XV – was Woodrow Wilson, on January 4, 1919, at the end of the First World War.

Wilson was arguably the first American president to blatantly ignore a papal request: Benedict XV asked for the meeting because he wanted to have the Holy See included in the Paris Peace Conference, which ended World War I. Wilson chose to cooperate with Italy instead, and the Holy See was excluded from the peace conference.

With the first Catholic president, a diplomatic detente

As the first Catholic president of the United States, John F. Kennedy, was careful to avoid any public appearance of favoring the Church. During the campaign he said that he believed in an America where the separation between Church and State is absolute, and “where no Catholic prelate would tell the president, should he be Catholic, how to act.”

It’s perhaps for this reason that he stalled the long process of rapprochement between America and the Vatican that began when his father went to Pius XII’s inaugural Mass.

Two years after taking office, JFK went to Europe and scheduled a meeting with John XXIII in 1963. However, the Good Pope died of stomach cancer between the announcement of the tour and the president’s arrival. Kennedy delayed his landing in Rome to arrive a day after the coronation of Pope Paul VI.

The meeting was labeled as “informal, low-key and unofficial,” publicly announced as “private” and not an official state visit.

But diplomacy was set aside when Kennedy was assassinated that November: The pope took the unprecedented step of inviting an American TV crew (ABC) into the papal apartments, from where he spoke of his sorrow at “so dastardly a crime.”

Lyndon B. Johnson: The UN trumps the U.S., Vietnam and an ego-filled gift

Two years after his meeting with Kennedy, Pope Paul VI became the first reigning pope to visit the United States.

Paul VI had been invited to address the General Assembly of the United Nations, and in order to do so, he left the fourth session of the Second Vatican Council to deliver one of his signature phrases: “No more war! War never again!”

Johnson wanted to meet the pope at the airport, but the pope refused: He was a guest of the UN, not the United States. Instead, they had a 50-minute meeting in the president’s suite at the Waldorf-Astoria.

On Dec. 23, 1967, the two met again, this time in the Vatican, as Johnson was returning from a presidential visit to Saigon. The stop was kept under wraps, even from Paul VI himself.

This meeting came after an exchange of messages over the Vietnam war, with the pope throwing the first stone: Paul VI asked Johnson to “increase even more your noble effort” to forge peace during a temporary truce following the New Lunar Year in early 1967. The president answered by saying that the U.S. is “prepared to talk at any time and place, in any forum, with the object of bringing peace to Vietnam; however, I know you would not expect us to reduce military action unless the other side is willing to do likewise.”

During their December meeting, Paul VI reiterated his objection to the Vietnam War and in return, Johnson gave the pope a memorable gift: A bronze bust of himself.

Nixon, the friendly and the acrimonious meetings

The first time President Richard M. Nixon met the pope was in 1969 in Rome. He and Paul VI spoke for 75 minutes, during which time they reportedly discussed the process of the Vietnam peace talks, at the time underway in Paris. They also talked about the use of nuclear energy for peaceful purposes and aid to poor nations.

A year later, however, their second meeting, also in Rome, was described as “less than pleasant, even acrimonious,” by Peter Hebblethwaite in his papal biography, Paul VI, the first modern pope.

The pope spoke without preamble of the impact of the Vietnam war. Underlining that Nixon was the “leader of a nation upon which weighs such a heavy share of responsibility for the present and for the future of the world,” Paul VI spoke of “the suffering which war inflicts not only on the combatants, but also on innocent persons, and on children who have no understanding even of the meaning of the word.”

The more power one has, the pontiff insisted, the bigger the responsibility in forging peace.

John Paul II: five presidents, Communism and the (culture) wars

On October 6, 1979, Pope John Paul II visited the White House. If there were contentious issues between him and President Jimmy Carter, history has chosen to forget them, at least officially.

In his personal notes – available in the National Archives – Carter focused on how each drew on their strong Christian beliefs as they discussed the human rights situations across the world, including China, South Korea, and the Middle East.

“As human beings each acting for justice in the present — and striving together for a common future of peace and love,” Carter said at the time.

Ronald Reagan first visited John Paul II in Rome was in 1982, a year after each suffered and survived assassination attempts, just weeks apart. They had much more than this in common, however, including the fact that both forgave their assailants and were both actors before changing careers.

It was under Reagan that the Holy See and the U.S. would forge official diplomatic ties.

But it was that first meeting that change the course of the encounters between the two global and moral superpowers: Their 50-minute chat was behind closed doors, as almost all visits by heads of states to popes have been ever since.

They also exchanged many letters, and their odd partnership reshaped Europe amid the Cold War and, many observers have noted, played a key role in bringing down the Soviet Union.

The first Bush to be president – George H. W .- had a somewhat less intense relationship with the Vatican than Reagan. If anything, the two Bushes were the U.S. presidents with whom John Paul II clashed the most, as he publicly and privately tried to dissuade them from starting wars.

Bush senior met the pope soon before the fall of the Berlin Wall, and again in 1991, following the First Gulf War that John Paul II had tried to prevent.

President Bill Clinton and John Paul II clashed over the “culture” war of abortion, a recurrent issue during their four encounters. The first time they met, in 1994 during the papal visit to Denver, John Paul II delivered a clearly pro-life message.

“If you want equal justice for all and true freedom and lasting peace, then, America, defend life,” he said to the crowd, president included. “All the great causes that are yours today will have meaning only to the extent that you guarantee the right to life and protect the human person.”

By the time the two saw each other again, this time in Rome, the Clinton administration had made it clear that they wanted to make abortion a “legally enforceable universal human right.”

Bush, Benedict, and Obama

Though abortion went on the backburner during the two terms – and six Roman trips – of Bush’s son, George W., the relationship between him and the popes – John Paul and Benedict XVI – was far from rosy due to the Iraq war, Afghanistan, and another pro-life issue: The death penalty.

When Bush welcomed Benedict in the White House in 2008, however, both chose to avoid talking about what divided them and focused instead on the common ground of protecting the unborn child.

Obama first shook hands with a pope on July 10, 2009, when he met Pope Benedict XVI in the Vatican. The German pontiff used his first-ever meeting with Obama to deliver a strong pro-life message, even pointedly offering him a copy of a recent Vatican document on bioethics.

During his second term, Obama traveled to Rome to meet with Francis, and though by the time the president left office the two were visibly in good terms – having cooperated on several issues, including Cuba, migration and climate change – after that first meeting the President and the Vatican had slightly different takes on the tenor of their discussions.

“In the context of bilateral relations and cooperation between Church and State, there was a discussion on questions of particular relevance for the Church in that country, such as the exercise of the rights to religious freedom, life and conscientious objection,” read the official Vatican statement about the meeting.

Talking to reporters, Obama said that these issues were “not a topic of conversation” with the pope, acknowledging instead that they had been discussed with the Vatican’s Secretary of State, Cardinal Pietro Parolin.

Francis and Trump: bridges and a wall

The Argentine pope and President Donald Trump disagreed on environmental policy and Trump’s proposed U.S.-Mexico border wall; however, their 2017 meeting in the Vatican was amicable – despite a picture of a frowny looking pope that went viral.

Francis is, after all, a man known for building bridges. It should come as no surprise if the same is said on Friday.