Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE has told colleagues he’s unsure if he can work with John Bolton, President Trump Donald John TrumpBubba Wallace to be driver of Michael Jordan, Denny Hamlin NASCAR team Graham: GOP will confirm Trump's Supreme Court nominee before the election Southwest Airlines, unions call for six-month extension of government aid MORE’s new pick for national security adviser, The New York Times reported.

Mattis reportedly told staffers he would find it difficult to work with Bolton prior to Trump’s announcement last Thursday that Bolton would replace H.R. McMaster.

Bolton is said to be an unpopular pick with both Mattis and White House chief of staff John Kelly John Francis KellyMORE.

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The president tweeted last Thursday evening that Bolton, the hawkish former George W. Bush administration official, will take over for McMaster on April 9.

McMaster, an Army lieutenant general, will retire from the military, a White House official said.

Bolton served as U.S. ambassador to the United Nations under the Bush administration, and as undersecretary of State in the years leading up to the Iraq War, and had a focus on preventing the spread of weapons of mass destruction.

Democrats have expressed concerns about Bolton and his past pro-war views.

Bolton will be Trump's third national security adviser. His first, Michael Flynn, resigned last year and has since pleaded guilty in special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation into Russian interference in the 2016 election.