One of the jobs of being a modern U.S. president is to make official state visits abroad. In the 21st century, presidents visit dozens of countries by the end of their respective terms. Yet during the 1800s, the idea of a U.S. president making even one or two international visits—let alone over 50—was unheard of.

Part of the reason early presidents didn’t leave the country has to do with the transportation available at the time. It took Woodrow Wilson, one of the first presidents to make an official visit abroad, nine days to sail to Europe in 1918. Four decades later, it only took Dwight D. Eisenhower nine hours to make the same trip by jet, notes scholar Richard Ellis in his book Presidential Travel.

But slow transportation wasn’t the sole reason 19th century presidents stayed in the U.S. As Ellis—a professor of politics at Willamette University—writes in his book, there was also a strong taboo against presidents going abroad and associating with European monarchs.

“The taboo against foreign travel by a president owed its staying power to the continuing hold that the republican fear of monarchical pomp and power had on the American imagination,” he writes. “A president who traveled abroad, Americans feared, would be invited to visit palaces and courts, to exchange pleasantries and genuflections with kings and queens.”

Tizoc Chavez, a political science professor at Vanderbilt University, says that Americans weren’t necessarily worried that the president would want more executive power after meeting with kings; but rather that, in meeting with monarchs, “he would degrade America’s image as being free and separate from the old world.”

Domestic travel, which allowed the president to connect with voters, seemed much more appropriate than trips to other places. In fact, the custom of not leaving the continental U.S. was so ingrained, Chavez says, that Ulysses S. Grant apparently thought there was some kind of law that presidents couldn’t go abroad.

This all began to change in 1906, when Teddy Roosevelt visited the construction of the Panama Canal. It was the first time a U.S. president had made a diplomatic trip to another country, and it paved the way for presidential travel to play a role in international relations.

Roosevelt’s trip wasn’t too controversial because he was overseeing a U.S. project (it was also his only trip abroad as president). In contrast, Republicans in Congress greatly criticized Democrat Woodrow Wilson’s two-month trip to Europe at the end of World War I. According to them, it was a sign that Wilson was focusing too much on foreign issues at the expense of his duties to the country. Chavez says Congress even tried to pass a law transferring the president’s power to the vice president when Wilson was abroad (it didn’t pass).

Even Franklin Delano Roosevelt’s attendance of the 1945 Yalta Conference—where the Allied leaders met with Stalin—was criticized, though mostly after he’d died and the Cold War had begun. This is because some people viewed it as an instance in which “a weak sickly Roosevelt sold out eastern Europe and helped lead to the Cold War,” Chavez says.

Although the United States’ global role expanded after World War II, international diplomacy still wasn’t seen as a necessary part of the president’s duties; and Yalta became one more example of why presidents should probably stay at home. Presidents over the next couple of decades carefully framed their international visits as goodwill missions, not negotiating trips.

Chavez says this situation slowly evolved over time. One of the major moments in this transformation was Richard Nixon’s trip to China in 1972, which his administration still framed as an opening of relations rather than an act of diplomacy.

In the “post-WWII, Cold War environment,” he says, “the idea that America is this leader of the free world and the president needs to be active” in the world helped make presidential trips abroad more normal. Instead of a distraction from his Constitutional duties, diplomacy has now evolved into something that is, itself, a presidential responsibility.

Gradually, Americans became “more comfortable with this idea that our president needs to be doing this,” Chavez says. “He can’t just stay home.”