It would be funny if it wasn’t so Trumpy.

Hosting the Japanese prime minister, Sanae Takaichi, in the Oval Office on Thursday, Donald Trump could not resist mocking Japan about its 1941 attack on Pearl Harbor during the second world war.

After a series of questions about the current conflict in Iran, the US president was asked by a Japanese reporter: “Why didn’t you tell US allies in Europe and Asia and Japan about the war before attacking Iran?”

Trump replied: “One thing you don’t want to signal too much, you know, when we go in, we went in very hard and we didn’t tell anybody about it because we wanted surprise. Who knows better about surprise than Japan?”

There was laughter in the room but the president had not finished. He asked mischievously: “Why didn’t you tell me about Pearl Harbor?”

Suddenly the laughter died away. Takaichi’s eyes widened and she shifted in her chair as Trump evoked the moment that drew the US into the second world war.

The Japanese attack on the US naval base in Pearl Harbor, Hawaii, took place on 7 December 1941, nearly five years before Trump was born. It killed 2,390 Americans and the US declared war on Japan the next day. Then president Franklin Delano Roosevelt called it “a date which will live in infamy”.

The US defeated Japan in August 1945, days after atomic bomb attacks on Hiroshima and Nagasaki killed hundreds of thousands of civilians.

Trump supporters relished his off-the-cuff response. His son Eric posted on the X social media platform: “One of the great responses to a reporter in history!”

Critics were less impressed. Journalist Mehdi Hasan wrote: “I’m sorry, but this is legit hilarious. If only he wasn’t the president and just a character on TV. We could laugh our heads off without any sense of unease, dread, or embarrassment.”

It was not Trump’s first awkward moment regarding the war. Last year, when Germany’s chancellor, Friedrich Merz, brought up 6 June as D-day, Trump responded that it was “not a pleasant day” for the chancellor.

Merz replied: “Well in the long run, Mr President, this was the liberation of my country from Nazi dictatorship.”

Trump has repeatedly complained on camera and online this week that US allies, including Japan, did not heed his request to help safeguard the strait of Hormuz after he launched war on Iran. On Thursday, he declared the help wasn’t needed, but made other comments indicating he still expected help, saying: “it’s appropriate that people step up.”

For Japan to send its Self-Defense Forces abroad is politically sensitive for the officially pacifist nation, as many voters support the US-imposed 1947 constitution that renounces war.

Takaichi later told reporters they agreed that ensuring the safety of the strait was of the utmost importance, but she gave Trump a detailed explanation of the actions Japan can and cannot take under its law.

The meeting appeared to highlight once again Takaichi’s ability to charm Trump, following a similarly friendly meeting in Tokyo in October during which she said she would nominate him for the Nobel peace prize. At one point she said: “I firmly believe that it is only you, Donald, who can achieve peace across the world.”