On Sunday, President Donald Trump announced on Twitter that he has named Patrick Shanahan acting secretary of defense. Shanahan will replace James Mattis, who resigned in protest after Trump announced he planned to pull U.S. troops out of Syria. Mattis will stay on until Jan. 1, and then Shanahan takes over. Shanahan will lead the Pentagon until Trump settles on a permanent replacement.

But who is the new secretary of defense? Five things to know about Shanahan:

1. He has no military experience

Shanahan, 56, did not serve in the military and has little foreign policy and government experience. That makes him an unusual pick for secretary of defense. Shanahan is deputy secretary of defense, handling day-to-day oversight of the agency and managing its budget.

In an interview with Defense News, Shanahan said he believes his job is to “operationalize the National Defense Strategy, driving systemic change – rewiring the organization to increase our performance on lethality, alliances, and reform.”

“Too often we focus on process, or budget, or level of effort,” he said. “The Pentagon should focus on outcomes and outputs – our performance. This focus on performance should drive us to field unmatched lethality, execute on our modernization plans and achieve this affordably.”

2. He was a longtime Boeing exec

Before his appointment as deputy defense secretary, Shanahan spent more than three decades at Boeing. He held a variety of senior vice president positions at the aerospace company over his career; The Puget Sound Business Journal once called him Boeing’s “fix-it” man. He oversaw work on new airliners and missile and laser defense systems.

According to his Department of Defense bio, Shanahan holds two advanced degrees from MIT: a masters of science in mechanical engineering and an MBA from MIT’s Sloan School of Management.

3. His father taught him about “service before self”

Shanahan’s father, Mike, was a Vietnam veteran who served in the U.S. Army. At an October 2017 event celebrating the release of the documentary “The Vietnam War,” Shanahan said that “growing up, my understanding of the war came from my dad, his friends and the few stories they would share.”

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In his confirmation hearing before the Senate Armed Services Committee, Shanahan said his father raised him and his younger brothers with the American ideology of “service before self.”

4. Trump likes him

Trump and Mattis had a contentious relationship, according to multiple reports, but the president likes Shanahan.

According to The Washington Post, Trump often complained to aides that Mattis did not “share his enthusiasm for negotiating defense contracts.” But Trump likes that Shanahan has taken a special interest in those matters. Additionally, The Hill reported that Shanahan frequently visited Trump and Vice President Mike Pence as the project leader within the Pentagon of the proposed Space Force military service.

5. Sen. John McCain was not a fan

Though Trump and Pence seem to be in the Shanahan fan club, Sen. John McCain – long considered one of Washington’s most respected voices on all things related to the military – was not.

In 2017, the Arizona Republican, who died in August, threatened to block Shanahan’s appointment after Shanahan’s response to a question about providing arms to Ukraine and his lack of Pentagon experience, according to CNN.

More:Read Defense Secretary Jim Mattis' resignation letter

McCain, then chairman of the Armed Services Committee, which oversaw hearings on Shanahan’s appointment, was vocal in his criticism of defense contractors taking senior jobs at the Pentagon. And he wasn’t happy that someone with no military experience was coming in to lead the Pentagon.

Shanahan was confirmed by the Senate 92-7.