Some notable moments in the nearly 100-year history of meetings between the pope and American presidents:
President Woodrow Wilson and Pope Benedict XV held the first meeting at the Vatican on Jan. 4, 1919, after World War I.
President John F. Kennedy's 1963 meeting with Pope Paul VI at the Vatican was historic: The first Roman Catholic president of the United States saw the Roman Catholic pontiff only days after the pope's coronation. Kennedy, who struggled against anti-Catholic bias during his presidential campaign, only shook hands with the pope rather than kissing his ring, the usual practice for Catholics.
Pope Paul VI's trip to New York in October 1965 presented protocol problems. President Lyndon B. Johnson wanted to see him, but the pontiff was chief of a state not officially recognized by the U.S. The solution: Johnson flew to New York for dinner at the apartment of his friend Arthur Goldberg, the U.S. ambassador to the United Nations, and the pontiff was welcomed to Johnson's suite at the Waldorf-Astoria Hotel the next day.
Johnson had a penchant for somewhat odd papal presents. At that 1965 meeting, his gifts to Paul included a silver-framed, autographed photograph of himself. At their next meeting two years later at the Vatican, Johnson presented the pontiff with a foot-high bust of himself.
President Jimmy Carter hosted the first White House visit by a pope. Pope John Paul II was greeted at Andrews Air Force Base outside Washington by Vice President Walter Mondale. The White House visit featured 10,000 guests, split between separate arrival and departure ceremonies on the North and South Lawns.
President Ronald Reagan had trouble keeping his eyes open on his first visit to the Vatican in 1982. Reagan's head bobbed and his eyes repeatedly closed for seconds at a time while John Paul talked solemnly of crises in the Falkland Islands and Lebanon. The incident came during a 10-day European trip with a schedule unusually packed for Reagan. But it fed already rampant talk that the 71-year-old president wasn't physically up to the job.
Reagan sparked controversy over the separation of church and state by establishing in 1984 formal U.S. diplomatic relations with the Vatican, long a desire of the Holy See. Shortly afterward, Reagan and John Paul had a quick meeting during separate refueling stops at the airport in Fairbanks, Alaska, in May 1984. The pope had stopped there on his way to Seoul, South Korea; Reagan was on his way back from China. The Alaska stop generated tons of excitement, along with scores of T-shirts that said: "The Pope Meets the Dope."
For his last of four meetings with Pope John Paul, President Bill Clinton flew to St. Louis to greet the pontiff as he began a U.S. tour. Back in Washington, the Senate was in the throes of its impeachment trial against Clinton, and John Paul said "America faces a time of trial." But it was generally assumed that the pontiff, who also challenged Americans to "a higher moral vision," was speaking about his long-running, unusually public and sharp dispute with the pro-abortion rights Clinton.
At President George W. Bush's last meeting with John Paul, at the Vatican in June 2004, he presented the pope with the Medal of Freedom, the nation's highest civilian honor. The pontiff responded by reading a statement about his "grave concern" over events in Iraq, where the U.S.-led war had been going on for just over a year.
After John Paul died in 2005, Bush became the first sitting U.S. president to attend a papal funeral.
At his first audience with John Paul's successor, Pope Benedict XVI, at the Vatican in June 2007, Bush's overly casual behavior was noted by many Italians and Vatican watchers. He addressed the pope as "Sir," rather than the customary "Your Holiness." and leaned far back in his chair with one leg thrown informally over another, instead of adopting the ramrod-straight physical posture more commonly seen in the pontiff's presence. Rome's ANSA agency flashed a "Gaffe Presidente" headline.
Bush committed several firsts for Benedict's first U.S. tour as pope. The president and first lady Laura Bush went to Andrews Air Force Base to welcome the pontiff. Bush invited 13,500 people, the largest crowd of his presidency, to the White House for Benedict's arrival ceremony on the South Lawn. Bush also hosted a dinner honoring the pope, though Benedict did not attend.
Benedict's White House visit came on his 81st birthday, and the crowd that had gathered on the South Lawn sang "Happy Birthday" twice to the German-born pope. He also was presented with a tiered birthday cake baked by the White House pastry chef.
President Barack Obama and Pope Francis met for the first time in March 2014 at the Vatican. Afterward, the Vatican and the White House offered differing accounts of what the leaders discussed during their nearly hour-long meeting. Obama stressed their common ground on fighting inequality and poverty while Vatican officials emphasized the importance to the Roman Catholic Church of "rights to religious freedom, life and conscientious objection." That point referred to a major disagreement over mandatory contraceptive coverage under Obama's health care law.
Like Bush, Obama is personally welcoming the pope. Obama and first lady Michelle Obama decided to be at the foot of the airplane stairs at Andrews Air Force Base to greet Francis when he arrives in the U.S. for the first time.
The White House has invited some 15,000 people to Francis' arrival ceremony on the South Lawn on Wednesday morning.