President Trump announced that Defense Secretary Jim Mattis will leave the administration nearly two months earlier than expected, and he will be replaced by Deputy Defense Secretary Patrick Shanahan.

“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!” the president said in a Sunday tweet.



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! — Donald J. Trump (@realDonaldTrump) December 23, 2018



Shanahan has, in recent weeks, played a key role developing the Pentagon's budget proposals under changing demands by President Trump. DOD initially sought $733 billion for fiscal year 2019.

Trump asked all federal agencies to take a 5 percent budget cut, which would have put DOD at $700 billion — a level military brass and Pentagon planners pushed back against.

Trump recently reversed himself and reportedly is ready to provide the military $750 billion for fiscal year 2019. That should save Shanahan, as acting secretary, from having to made a series of unpopular cuts to weapons programs and other DOD initiatives.

Mattis resigned from the Trump administration on Thursday in protest of the president’s abrupt announcement to withdraw all U.S. forces from Syria.

Mattis wrote in his resignation letter that he felt he could no longer continue to execute Trump's policies, and cited the importance of what he called America's "unique and comprehensive system of alliances and partnerships."

"Because you have the right to have a secretary of defense whose views are better aligned with yours ... I believe it is right for me to step down from my position,” Mattis said.

Mattis had said in his letter his last day would be on Feb. 28 to ensure a smooth transition to his successor, but Trump said Shanahan will start next week.

Trump had nominated Shanahan to the Pentagon’s second-highest civilian position in March 2017, and he was confirmed by the Senate in July 2017.

Shanahan, a graduate of the University of Washington and Massachusetts Institute of Technology, is a former executive with Boeing who first joined the company in 1986.

Throughout his career with the company, he held management roles with Boeing's Missile Defense Systems, first beginning in 2004, as well as with the 737, 747, 767, 777 and 787 commercial airline programs.

Shanahan is also known for playing a key role in spearheading the recovery of Boeing's 787 program.

Shanahan will assume command of the Defense Department just days before Democrats assume the House majority. The incoming House Armed Services Committee chairman, Rep. Adam Smith, D-Wash., is much more skeptical of military spending than the current panel head, Rep. Mac Thornberry, R-Texas.

Smith can be expected to rake DOD officials over the coals on weapons systems requests, likely a headache for Shanahan as long as he serves as acting secretary.

Defense News reported earlier this week that Shanahan was poised to take over the Pentagon — where he has spent the past year-and-a-half reforming its internal processes.

“He’s not a policy or geopolitics guy. He’s a business guy,” a source told the outlet. “But he’s spent the last year learning from the best. And Shanahan is known for having a good relationship” with Trump and Vice President Mike Pence.

Shanahan began focusing on what “long-lead modernization issues” had to be addressed at DOD, and the situations in both the Middle East and North Korea. He assumed the position days after Trump threatened Pyongyang with “fire and fury.”

Shanahan was particularly focused on getting rid of duplicative systems, consolidating functions, and saving costs.

As deputy defense secretary, Shanahan also reportedly took the lead on Trump’s sought-after Space Force.

“The people that are going to do space development, their principle job is to ensure horizontal integration across the department. It’s less about the space mission than it is this is a once in a lifetime opportunity to drive standards and integration,” Shanahan told Defense News in November. “This isn’t technology. This is about the network."

But Shanahan, reportedly not close with Mattis, does have critics.

Several officials told Defense News that Shanahan was “too focused on having meetings,” and described him as having a “combative personality.”

Asked about criticisms in an email earlier this week, Shanahan downplayed them as “completely normal and expected with instituting systemic change.”

“It is also natural that people are defensive when leaders begin focusing on performance rather than process,” he wrote.