More: "Hey, John, what's this all about? What's this a tour of?" Mr Trump reportedly asked John Kelly, his then-chief of staff, when they took a private tour in 2017 of the USS Arizona Memorial, a ship commemorating the surprise Japanese attack on Pearl Harbor during the Second World War.

"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," write the authors, who quote a former White House adviser concluding the US president was "dangerously uninformed".

During a meeting with Narendra Modi, the Indian prime minister's "eyes bulged out in surprise", the Washington Post reporters claim, when Mr Trump told him: "It's not like you've got China on your border."

China and India in fact share more than 2,000 miles of common border.

Mr Modi's expression "shifted from shock and concern to resignation", with aides telling the authors the Indians "took a step back" in their diplomatic relations with the US following the meeting.