In the book A Very Stable Genius, the two Washington Post and Pulitzer prize winning authors recount a meeting between Trump and Indian Prime Minister Narendra Modi. Trump remarked, “It’s not like you’ve got China on your border,” which prompted “shock and concern” from the PM, which turned to “resignation.” The border between India and China, called the Border of Actual Control, is over 2,500 miles long. The two countries have multiple border disputes and have engaged in three conflicts between 1962 and 1987.

Trump also previously thought that the neighboring countries of Nepal and Bhutan were in India, pronouncing them as “nipple” and “button.”

India Today – Donald Trump didn’t know India-China share border, PM Narendra Modi was shocked: New book
New Zealand Herald – Donald Trump reportedly didn’t know India and China shared a border
Wikipedia – China-India Relations