United States President Donald Trump once dismissed the threat posed by China to India by telling Prime Minister Narendra Modi that the two countries do not even share a border, The Washington Post reported on Wednesday, citing a new book. It is not clear when the conversation took place.

The book, titled A Very Stable Genius: Donald J. Trump’s Testing of America, was written by Pulitzer-winning journalists Philip Rucker and Carol D Leonnig.

According to the book, Trump told Modi during a meeting, “It’s not like you’ve got China on your border.” In response, “Modi’s eyes bulged out in surprise” and his “expression gradually shifted, from shock and concern to resignation”, the authors claimed. A Trump aide told the two journalists that Modi probably left that meeting and said, “This is not a serious man. I cannot count on this man as a partner.”

India shares a 3,488-km border with China.

“The Indians took a step back in their diplomatic relations with the United States,” the aide said after the meeting.

In another anecdote described in the book, Trump appeared to not know about the history behind the Pearl Harbor attack. “Hey, John, what’s this all about? What’s this a tour of?” Trump asked his then-Chief of Staff John Kelly before a tour of a memorial of the 1941 attack.

The authors write: “Trump had heard the phrase ‘Pearl Harbor’ and appeared to understand that he was visiting the scene of a historic battle, but he did not seem to know much else.”

A former senior White House adviser told the authors that Trump was “at times dangerously uninformed”.

Questions have often been raised about Trump’s knowledge of world geography. In the past, he has called Belgium “a beautiful city”, even though it is a country. He has reportedly also expressed surprise that Nepal and Bhutan were countries, rather than parts of India, and he pronounced them as “nipple” and “button”.