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 bid goodbye to Pentagon employees Monday, writing in his farewell memo that the department “is proven to be at its best when the times are most difficult.”

Mattis, who resigned earlier in December, also insisted that the Pentagon’s civilian and military leadership remains “in the best possible hands,” one day before Patrick Shanahan Patrick Michael ShanahanHouse Armed Services chairman expresses confidence in Esper amid aircraft carrier coronavirus crisis Boeing pleads for bailout under weight of coronavirus, 737 fallout Esper's chief of staff to depart at end of January MORE, his deputy, takes over as acting secretary of Defense.

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Mattis tendered his resignation on Dec. 20, shortly after President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE abruptly announced a decision to withdraw U.S. troops from Syria and amid speculation he could do the same in Afghanistan. Mattis’s resignation letter cited policy disagreements with the president, and his exit rattled Republicans, who had long seen him as a stabilizing force within the administration.

Trump has since double down on his decision to withdraw American troops from Syria, which has been widely criticized, including by Republicans in Washington. Trump suggested on Twitter early Monday that he had been unfairly attacked by the media and “some failed Generals” for the move.

“If anybody but Donald Trump did what I did in Syria, which was an ISIS loaded mess when I became President, they would be a national hero. ISIS is mostly gone, we’re slowly sending our troops back home to be with their families, while at the same time fighting ISIS remnants,” Trump wrote.

Mattis initially planned to resign in late February, allowing Trump time to find an adequate replacement to helm the Pentagon. However, Trump announced days after Mattis’s resignation that Shanahan, a former Boeing executive with little government experience, would replace him temporarily at the start of 2019.

Trump's decision appeared to have been influenced by the widespread criticism of him after Mattis's announcement.

In his farewell memo, Mattis quoted a February 1865 letter from President Lincoln to Gen. Ulysses S. Grant that read, “Let nothing which is transpiring, change, hinder, or delay your military movements, or plans.”

Mattis went on to urge his colleagues to “keep the faith in our country and hold fast, alongside our allies, aligned against our foes.”