President Donald Trump will replace outgoing Defense Secretary James Mattis with his top deputy, Patrick Shanahan, from Jan. 1, ending the former general’s tenure two months before his planned exit from the Pentagon.

Trump announced the replacement on his twitter page :

I am pleased to announce that our very talented Deputy Secretary of Defense, Patrick Shanahan, will assume the title of Acting Secretary of Defense starting January 1, 2019. Patrick has a long list of accomplishments while serving as Deputy, & previously Boeing. He will be great!

Mattis announced his resignation on Thursday in a letter underscoring his policy differences with the president but saying he’d stay on the job until Feb. 28. The retired Marine Corps general quit after Trump announced his plans to withdraw U.S. troops from Syria.