Washington — Secretary of Defense Jim Mattis will be leaving his post two months earlier than initially reported.

Mattis resigned Thursday in protest over President Donald Trump's abrupt decision to withdraw U.S. troops from Syria. Trump initially framed Mattis' departure as a retirement.

Both Mattis and Trump indicated just three days ago that he would be staying on until the end of February in order to give time for a replacement Secretary of Defense to be nominated and confirmed.

But Trump announced Sunday morning via Twitter that Deputy Secretary of Defense, Patrick Shanahan, would be taking over as the Acting Secretary of Defense, starting January 1.

"Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!," Trump tweeted.