One of the jobs of being a modern U.S. president is to make official state visits abroad. Both George W. Bush and Barack Obama visited dozens of countries during their respective terms, and Donald Trump is poised to do the same. 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.”