In a anecdote from a new book by Washington Post reporters Philip Rucker and Carol Leonning, President Donald Trump is revealed to have been ignorant of what happened on December 7, 1941.

The book, A Very Stable Genius, takes into account hundreds of hours of interviews for more than 200 sources.

The day marked one of the biggest attacks on U.S. soil that led to the country’s involvement in World War II. On a trip to Hawaii for a private tour of the USS Arizona Memorial, Trump reportedly asked former White House Chief of Staff John Kelly, “Hey, John, what’s this all about?“ Kelly is a retired Marine Corps general, and then later led the Department of Homeland Security before being named Trump’s Chief of Staff.

“He was at times dangerously uninformed,” a former senior White House official told the reporters.

However, Trump has previously shown knowledge of the topic. He tweeted in 2016: “Does President Obama ever discuss the sneak attack on Pearl Harbour while he’s in Japan? Thousands of American lives lost.”

The authors of the book reported that although Trump did understand that Pearl Harbor was significant in American history, he was short of knowing details. His understanding was that he was going to visit a historic battle site.

Pearl Harbor was a military strike against the United States by Japan. 2,400 US service members and civilians were killed.

The memorial has been in Hawaii since 1962 and stands above the original USS Arizona.