Bob Woodward said former Defense Secretary James Mattis agrees with the veteran political journalist’s widely shared warnings about Donald Trump’s erratic temperament, outlined in his new book.

Woodward appeared on The Bulwark Podcast Thursday to promote his book War. Upon the book’s Oct. 15 release, Woodward said Mattis sent him an email seconding his thoughts about Trump’s worrisome behavior during his presidential term.

”He thinks the book is important,” said Woodward about Mattis to host Tim Miller. ”He believes it’s true. And it was a kind of, you know, ‘Hey, I understand this.’ It was the strongest endorsement.”

Mattis served a stint in Trump’s administration before resigning in protest following a decision to withdraw U.S. forces from Syria. There had also been a series of disagreements over Trump’s approach to international alliances, reported The Atlantic.

“I’ve been informed by four decades of experience, and I just couldn’t connect the dots anymore,” said Mattis about serving under Trump.

Following his resignation, Mattis continued to issue statements objecting to Trump’s behavior. Following the Jan. 6 attack on the U.S. Capitol building, Mattis accused Trump of Trump of using his presidency to “destroy trust in our election and to poison our respect for fellow citizens.”

In his book, Woodward recounts that Mattis slept in his gym clothes because he was so worried that Trump would launch a nuclear strike against North Korea in the middle of the night.

In another show of concern, Woodward said Gen. Mark Milley, the former chairman of the Joint Chiefs of Staff, called Trump “the most dangerous person ever.”

Milley added that Trump was “fascist to the core.”