A week after UN Ambassador Nikki Haley announced she was standing down, the president suggested the 68-year-old former general could be the next to go.
During an interview with CBS, the president was asked if Mr Mattis – with whom he has at times reportedly had a strained relationship – was going to leave.
“I don’t know. He hasn’t told me that,” he said. “I have a very good relationship with him. I had lunch with him two days ago. I have a very good relationship with him. It could be that he is. I think he’s sort of a Democrat, if you want to know the truth.”
He added: “General Mattis is a good guy. We get along very well. He may leave. I mean, at some point, everybody leaves. Everybody. People leave. That’s Washington.”
There has been considerable speculation about how long Mr Mattis, a one time Marine Corps general who commanded the 1st Marine Division during the 2003 invasion of Iraq, would remain in Mr Trump’s government. The two men have differed on issues ranging from North Korea policy to the use of torture and transgender people serving in the military.
Speculation intensified last month with the publication of Bob Woodward’s Fear: Trump in the White House, which claimed the former military officer had described Mr Trump as an idiot and told colleagues the president “acted like – and had the understanding of – a fifth- or sixth-grader”.
Mr Mattis, who was in India when excerpts of the book were first published, denied he had ever made such remarks.
“The contemptuous words about the president attributed to me in Woodward’s book were never uttered by me or in my presence,” he said in a statement. “While I generally enjoy reading fiction, this is a uniquely Washington brand of literature, and his anonymous sources do not lend credibility.”
Back at the Pentagon, Mr Mattis told reporters: “I wouldn’t take it seriously at all. Of course I don’t think about leaving, I love it here.”
The departure of Mr Mattis would be a blow for those in the country who have viewed him as a moderating force on Mr Trump. Mr Woodward quotes one scene in which the president reportedly told the defence secretary to “f***ing kill” Bashar al-Assad after the Syrian leader was accused of using chemical weapons in an attack on civilians in April 2017.
The book said Mr Mattis told the president that he would get right on it. But after hanging up the phone, he told an aide: “We’re not going to do any of that. We’re going to be much more measured.”
Following the publication of the book, a number of senior Republicans spoke out in defence of Mr Mattis.
“The president does not ask my advice. If he were to ask my advice, I’d say keep secretary Mattis there as long as you possibly can,” said congressman Mac Thornberry, chairman of the house armed services committee.
“I think he’s been – is – a great secretary of defence, who has tremendous credibility on Capitol Hill, around the world. I think he’s done a really great job, and I would do everything I could to keep him there as long as possible.”
In the interview with CBS, Mr Trump said there were some people in his administration he is “not thrilled with”. He denied accusations of chaos in the White House as “fake news”.
But he added: “I’m changing things around. And I’m entitled to. I have people now on standby that will be phenomenal. They’ll come into the administration, they’ll be phenomenal.”
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