At an otherwise routine Oval Office meeting on Thursday, Donald Trump made a joke about Pearl Harbour directly to Japan's prime minister Sanae Takaichi, invoking the 1941 surprise attack that killed 2,400 Americans and drew the United States into the Second World War. Takaichi widened her eyes, said nothing, and moved on. For eight decades, American presidents have avoided the subject in Japanese company, understanding that the alliance depends in part on leaving certain histories alone. Trump, characteristically, did not.