Defense Secretary James Mattis said that President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE acted like and had the understanding of “a fifth- or sixth-grader” when it comes to the Korean peninsula, Bob Woodward’s forthcoming book reveals, according to The Washington Post.

The Post report on Tuesday recounts a National Security Council meeting on Jan. 19 in which Trump questioned U.S. government spending in the region, and played down the significance of having a big U.S. military presence on the Korean peninsula.

Trump also appeared to disregard the significance of a special intelligence operation set up in Korea that would allow the U.S. to detect a missile from North Korea in seven seconds, much faster than 15 minutes from Alaska, the Post reported, citing Woodward’s upcoming book, “Fear: Trump in the White House.”

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“We’re doing this in order to prevent World War III,” Mattis reportedly told President Trump.

After the meeting, “Mattis was particularly exasperated and alarmed, telling close associates that the president acted like — and had the understanding of — ‘a fifth- or sixth-grader,’” according to the Post's account of the book.

Woodward’s book, which was obtained by the Post, is set to be released next week. The book “reveals in unprecedented detail the harrowing life inside President Donald Trump’s White House and precisely how he makes decisions on major foreign and domestic policies,” according to Amazon.

Woodward, who is renowned for his work on the Watergate investigation, also wrote books on the previous two presidential administrations.