U.S. President Donald Trump began his presidency surrounding himself with military generals in key Cabinet posts. But his pick for acting defense secretary is a sharp departure from that mold.

Patrick Shanahan, who became the acting U.S. secretary of defense Tuesday, takes the helm of the country’s massive military machine at a critical time.

Shanahan is moving up from his position as deputy defense secretary to the top spot following the resignation of Jim Mattis, who quit over long-running — and ultimately irreconcilable — disagreements with Trump.

Little known outside business and Washington circles, Shanahan will be thrust onto the world stage at a time of tumultuous changes and unpredictable foreign policy moves under Trump.

The president last month ordered the withdrawal of all U.S. forces from Syria and, according to U.S. officials, a 50-percent drawdown in Afghanistan — momentous decisions that have left lawmakers and international allies fearing for what comes next.

Shanahan must now oversee the U.S. withdrawal from Syria and play a key role in helping Trump decide the pace of the pullout.

Another consideration he must weigh is what, if any, protections should go to Kurdish forces that the U.S. military had been backing to fight Islamic State jihadists.

While Shanahan has not always had day-to-day involvement in thorny geopolitical issues like this, Pentagon officials say he is ready to tackle them.

“He’s had 18 months to observe all of these operations, he’s been in all of the meetings, he’s been in all the discussions, he’s briefed the White House,” said Shanahan’s spokesman, Lieutenant Colonel Joe Buccino.

A Washington state native, the 56-year-old Shanahan spent more than three decades at Boeing, where he became senior vice president, worked on missile defense systems and was general manager of the 787 Dreamliner program.

Trump announced Shanahan’s appointment in a Dec. 23 tweet that also brought Mattis’ planned departure date forward by two months.

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

Despite a lack of foreign policy or military experience, Shanahan’s executive and technical knowledge made him well suited for the Pentagon’s No. 2 position, which focused on running day-to-day operations inside the building while Mattis took a more public role and focused on operations and geopolitics.

Shanahan’s background is in mechanical engineering, with two advanced degrees from the Massachusetts Institute of Technology.

In his time as deputy, the soft-spoken Shanahan played a key role in helping Mattis and others develop vital policy documents such as the National Defense Strategy, a sweeping review that highlights a new era of “Great Power competition” with Russia and China.

“In 2019, the National Defense Strategy remains our guide. America’s military strength remains our focus,” Shanahan said in a New Year’s message on Twitter, soon after arriving at the Pentagon on Wednesday.

A defense official added that Shanahan had told colleagues a major concern would be China.

“While we are focused on ongoing operations, Acting Defense Secretary Shanahan told the team to remember: China, China, China,” the official said.

Shanahan has also voiced support for Trump’s aim of creating a “Space Force,” a sixth branch of the military that still needs approval from the U.S. Congress.

Buccino pointed to the National Defense Strategy as a key area for Shanahan’s focus.

“He sees the National Defense Strategy as the blueprint, as the guide for all actions, directions, policies for the Pentagon,” Buccino said.

Before starting at the Pentagon in July 2017, Shanahan’s lack of foreign policy chops got him in hot water with the late John McCain, the senior Republican senator who led the Senate Armed Services Committee.

At his confirmation hearing, an irascible McCain said a lack of detail in some of Shanahan’s foreign policy answers was “almost insulting.”

McCain also expressed concerns that putting a former executive from a defense industry giant in the Pentagon could be akin to putting a fox “back in to the henhouse.”

McCain questioned how Shanahan would run day-to-day operations at the Pentagon despite his deep ties to the defense contractor Boeing, and then lit into him for something specific. In prepared answers to questions, Shanahan had said he would review whether the United States should send Ukraine weapons to defend itself against Russian-backed separatists. Shanahan amended his response to say that he would support the idea, but McCain still threatened to stop a vote on his confirmation.

“That’s not good enough, Mr. Shanahan,” McCain said. “I’m glad to hear you changed your opinion from what was submitted, but it’s still disturbing to me. It’s still disturbing to me after all these years that you would say that you would have to look at the issue. Have you not been aware of the issue?”

The exchange highlights the difficulties that Shanahan faces now. Some lawmakers, including Sen. Joni Ernst and Rep. Ted Lieu, have advocated for Trump to nominate Air Force Secretary Heather Wilson to take the defense secretary role on a permanent basis.

International allies will look to Shanahan, as they did to Mattis, for reassurances about America’s commitment to longstanding alliances — even as Trump assails partner nations for not paying more for their defense.

Shanahan must tread a careful line with the mercurial president, especially if he wants to get rid of the “acting” prefix of his job title.

Trump has shown an aversion to listening to dissenting advice, and has parted company with many of the top aides to have challenged him.

An official close to Shanahan said he would feel comfortable giving Trump an opposing opinion if necessary.

“His entire adult life, Shanahan has provided a dissenting voice to leadership when needed,” the official said.

“He would have no qualms doing that.”

Shanahan is the son of Vietnam veteran Mike Shanahan, who served in the U.S. Army. “He returned home and continued his selfless service to his fellow Americans with another 25 years in law enforcement,” Shanahan said, according to an October 2017 article on the Department of Defense’s website. “Growing up, my understanding of the war came from my dad, his friends and the few stories they would share.”

He also said that his father raised him and his younger brothers with the American ideology of “service before self.”

Shanahan has embraced government service since joining the administration, characterizing it as his opportunity to serve fellow Americans. At the Pentagon, he sometimes has “out-of-body” experiences, he said in September.

“I’m often asked about the transition to the department with an undertone, ‘Is it what you expected?’ ” Shanahan said in September at an Air Force Association conference. “I tell people, ‘It’s like breaking up with your longtime girlfriend and finding the love of your life.’”

At the Pentagon, Shanahan has also been involved in tense internal deliberations about how the Pentagon spends its money.

Shanahan predicted last year that there would be “screaming and yelling” over internal restructuring. Others in the Pentagon have said they are worried that Shanahan doesn’t understand the building’s culture and has struggled with high turnover on his immediate staff and tensions with the military services.

In public appearances, Shanahan has focused mainly on the subjects in which he has a leading role. Like Mattis, he also has underscored the importance of maintaining and expanding alliances abroad, a key point of disagreement between Mattis and the president.

According to U.S. media, Trump is especially angry over news coverage of Mattis’ stinging resignation letter that laid bare his fundamental disagreements with the president.

“Relationships aren’t monolithic. They’re complex,” Shanahan said in September. “We will agree in some places and disagree in others. Secretary Mattis has traveled to nearly 60 countries so far. That’s what I call commitment.”

Shanahan complimented Mattis during the same appearance for maintaining stability at the Pentagon and said working for him was like getting a Ph.D. in world affairs.

“Most people kind of think of him in the context of being a … military leader and, you know, motivator,” Shanahan said. “I’ve appreciated his real strength: He understands how to govern. He understands how government should work. He understands policy. He understands the law. He understands the value of relationships.”(SD-Agencies)