The president worries Mr Mattis is a Democrat “at heart,” according to the New York Times. Mr Trump has even reportedly started to call the former Marine general “Moderate Dog” instead of “Mad Dog,” a nickname Mr Mattis never liked in the first place.
Mr Trump is frustrated that the Pentagon chief is described as the adult in the room, White House aides and other officials told the Times, and wants someone running the department who is more of a vocal supporter, like US Secretary of State Mike Pompeo.
Bob Woodward’s new book, Fear, quotes Mr Mattis comparing Mr Trump to an elementary school student.
During a nuclear standoff with North Korea last January, the Defence Secretary told colleagues that the president was acting like a “fifth or sixth-grader,” according to the book.
Mr Mattis denied Mr Woodward’s reporting and also said he isn’t behind a recent anonymous op-ed in the New York Times by a senior administration official criticising the president.
But Mr Trump and Mr Mattis are currently at odds on policy.
They’ve reportedly clashed over NATO strategy, military exercises in South Korea, banning transgender people from serving in the military, and the effectiveness of Mr Trump’s move to withdraw the United States from the Iran nuclear deal.
Mr Mattis also reportedly rebuked a demand to stop family from going with troops deployed to South Korea, and a White House attempt to more closely supervise military operations.