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 wished U.S. troops a merry Christmas and worked Christmas Day 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 pushed the secretary out two months earlier than he planned to resign.

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“To those in the field or at sea, ‘keeping watch by night’ this holiday season, you should recognize that you carry on the proud legacy of those who stood the watch in decades past,” Mattis wrote in a holiday letter to U.S. troops.

“In this world awash in change, you hold the line. Storm clouds loom, yet because of you, your fellow citizens live safe at home.”

“Merry Christmas and may God hold you safe,” Mattis concluded.

#SecDef Mattis’s Holiday Letter to the Troops pic.twitter.com/xiwcpMpC4q — U.S. Dept of Defense (@DeptofDefense) December 25, 2018

CNN’s Pentagon correspondent Barbara Starr reported that Mattis was working at the Pentagon on Tuesday.

Defense Secretary James Mattis is in his third floor Pentagon office on Christmas Day working. — Barbara Starr (@barbarastarrcnn) December 25, 2018

On Thursday, Mattis tendered his resignation to President Trump after the president decided to withdraw all U.S. troops from Syria.

In his resignation letter, Mattis said his world views did not “align” with Trump’s, adding that “over four decades” of experience has taught him the value of alliances such as NATO and the anti-ISIS coalition and of standing strong against adversaries such as Russia and China.

In the letter, Mattis said he would depart at the end of February.

But on Sunday, after days of growing increasingly angry at the letter, Trump announced he was pushing Mattis out and installing his deputy, Patrick Shanahan, as acting secretary on Jan. 1.

In an interview with The Seattle Times published Monday, Mattis’s brother, Tom, said the secretary was not angry about being pushed out early. The Mattises hails from Washington state, where his brother and mother still live.

“He was very calm about the whole thing,” Tom Mattis told the newspaper of his Sunday call with his brother. “Very matter of fact. No anger.”

Tom Mattis added that his brother’s loyalty is to the Constitution and so he will “always give his best advice and speak truth to power — regardless of the consequences.”

He also said this is not the last Americans should expect to see of James Mattis.

“No one should assume that his service to his country will end,” Tom Mattis said. “And the manner of his departure is yet another service to the nation. It is the very definition of patriotism and integrity.”