Retired Gen. James “Mad Dog” Mattis formally stepped down from his position as secretary of defense on Monday with a memo to personnel at the Department of Defense.

“Our department’s leadership, civilian and military, remains in the best possible hands,” Mattis said in the memo, which was obtained by Time.

“I am confident that each of you remains undistracted from our sworn mission to support and defend the Constitution while protecting our way of life. Our department is proven to be at its best when the times are most difficult.”

President Donald Trump announced earlier in the month that he was withdrawing thousands of troops from Syria.

Mattis sent a resignation letter to the president shortly after that announcement.

Mattis detailed in the letter his belief in having a strong global presence — and that the president had the right to have a defense secretary whose views better aligned with his.

Related: Paul Gives Spirited Defense of Trump’s Decision to Pull Troops Out of Syria, Afghanistan

“So keep the faith in our country and hold fast, alongside our allies, aligned against our foes,” Mattis also said in the memo.

“It has been my high honor to serve at your side. May God hold you safe on the air, on land, and at sea.”

Mattis originally planned to stay until February 28 to give the administration time to find a replacement.

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But he is now scheduled to transfer responsibility to his deputy, Patrick Shanahan, on Monday evening.

Trump announced last week that Shanahan will become the acting secretary of defense starting January 1.

Trump is also facing additional opposition from within his own party over the decision to withdraw troops from Syria as well.

Trump went after his ongoing secretary of defense for not seeing countries that take advantage of the United States as a threat.

Brett McGurk, the special presidential envoy for the Global Coalition to Counter ISIL, resigned following the troop withdrawal announcement.

Sen. Lindsey Graham (R-S.C.) and other Republicans have also warned the president against the move; Graham met with the president over the weekend on this topic.

Related: Trump Faces GOP Backlash Over Troop Reductions in Syria and Afghanistan

Trump is still getting some support from party members for his decision. Sen. Rand Paul (R-Ky.) thanked the president for bringing troops home from the war-torn country during a speech on the floor shortly after the announcement.

Paul made the comments while speaking out against legislation that would prolong the conflict in Syria.

Trump later went after his ongoing secretary of defense for not seeing countries that take advantage of the United States as a threat. The president said he respects our allies but can’t stand countries that take advantage of their friendship with the United States, both in military protection and trade.

Mattis began the memo to personnel with a quote from former President Abraham Lincoln in a letter he sent to General Ulysses S. Grant, which reads: “Let nothing which is transpiring, change, hinder, or delay your military movements, or plans.”

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