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​​Deputy Defense Secretary Pat Shanahan is formally sworn into office by Defense Secretary Jim Mattis during a ceremony in the Pentagon in Washington, D.C., Aug. 14, 2017. Shanahan was confirmed by the Senate on July 18th and officially sworn into office on July 19. DOD photo by Army Sgt. Amber I. Smith.

President Trump on Sunday took to Twitter to announce that Defense Secretary Jim Mattis will be forced out of office two months early, appointing Deputy Defense Secretary Patrick Shanahan as his acting replacement effective Jan. 1.

Mattis announced Thursday he was resigning after almost two years as the leader of the Pentagon, saying in a two-page letter that he disagreed with several policy positions of the President and that Trump deserved a Defense Secretary he agreed with. 

Trump initially said Mattis would retire by the end of February, but that changed on Sunday with a Tweet saying, "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, in his letter, said he planned to stay until the end of February to ensure continuation in the office, participate in Congressional budget hearings, and attend a NATO defense minister's conference. 

Shanahan, a former Boeing executive, has served as deputy defense secretary since July 2017. He has overseen much of the department's acquisition programs and the development of the plan for a separate military service for space.