“Patrick has a long list of accomplishments while serving as Deputy,” the president tweeted. “He will be great!”

Defense Secretary Jim Mattis submitted his letter of resignation to Trump on Thursday, a day after the president announced he decided to “immediately” withdraw American troops from Syria.

In his blunt letter to the president, Mattis stated his last day would be Feb. 28. But Trump tweeted Sunday that Shanahan would step in as acting defense secretary on Jan. 1, greatly accelerating Mattis’ planned departure.

Trump, who reportedly hasn’t spoken to Mattis since Thursday, directed Secretary of State Mike Pompeo to alert the outgoing defense secretary of his updated departure date, The Washington Post reported Sunday.

Mattis, who opposed pulling U.S. troops out of war-torn Syria, wrote in his unusual resignation letter to Trump that because “you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position.”

“My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues,” he wrote.

Shanahan, a former Boeing executive, was confirmed by the Senate in 2017 by a 92-7 vote. Then–Senate Armed Services Committee Chairman John McCain (R-Ariz.), who died this year, threatened to keep his nomination from a vote at one point over Shanahan’s initial failure to voice support for providing military assistance to Ukraine in the face of Russian aggression.

Shanahan dodged specific questions regarding his views on Russia during his confirmation hearing, which mostly centered on his experience at Boeing. But he acknowledged that the U.S. needs to “deal” with Russian aggression in the future.

“I think the Russians are adversarial. I think through the whole of government we need to deal with their aggression ― their disruption to our interests. I at this point don’t have any specific recommendations,” he said.

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