During President Donald Trump's first foreign trip, all eyes will be on one meeting in particular: his one-on-one with Pope Francis. Although the two have never met, they already have plenty of baggage.
After the pope criticized Trump's plan to put up a wall on the southern border as not being Christian, Trump hit back and called the pope's comments disgraceful.
The "feud" was definitely eyebrow-raising in 2016, and that matters because the relationship between the American president and the pope is really important.
Believe it or not, the U.S. and the Holy See haven't always had true diplomatic relations. The first diplomatic mission was in 1848, but a permanent ambassador didn't actually exist until 1984 under President Ronald Reagan, despite the relationship being important throughout history.
For example, during WWII, then-President Franklin D. Roosevelt sent an envoy to Vatican City. With permission from the pope, Roosevelt set up a key diplomatic outpost in Italy.
During the Cuban missile crisis, Pope John XXIII wrote a pivotal statement imploring leaders to avoid a nuclear holocaust, something leader of the former Soviet Union Nikita Khrushchev called the only "the only gleam of hope." This happened after back-channel talks between the Vatican and White House.
But it was Reagan who changed the way popes and presidents interact. The two teamed up to push back against Soviet communism in the '80s, with Pope John Paul II using moral persuasion in tandem with everything the United States was doing.
The alliance isn't the same as our other bilateral partnerships. A big part of it hinges on the relationship between the president and the pope. It's all about diplomacy and influence, or soft power.
And the future of that relationship depends on this trip and how much the president prioritizes diplomacy in general.