President Trump’s new acting defense secretary comes to the job with deep experience in special operations, including the hunt for Osama bin Laden, but has never served in the Pentagon’s most senior ranks and only recently became the director of the National Counterterrorism Center.

Christopher Miller’s rapid ascension to acting Pentagon chief was announced by Trump on Monday as he fired Defense Secretary Mark T. Esper, who fell out of favor after disagreeing with the president over his threat in June to use active-duty military forces to quell unrest in America’s streets.

“I am pleased to announce that Christopher C. Miller, the highly respected Director of the National Counterterrorism Center (unanimously confirmed by the Senate), will be Acting Secretary of Defense, effective immediately,” Trump tweeted. “Chris will do a GREAT job! Mark Esper has been terminated. I would like to thank him for his service.”

Although rumors of Esper’s departure had swirled for months, Trump had held back on firing him out of concern that it could hurt his reelection chances, according to several officials familiar with the president’s thinking, who spoke on the condition of anonymity because of the sensitivity of the issue.

But with Trump losing last week and refusing to concede, there was seemingly little to stop him, despite concerns that ousting a defense secretary during a presidential transition could be damaging to national security.

Into that void steps Miller. Along with his dark suit, he wore a face mask as he approached the Pentagon on Monday, in keeping with the Defense Department’s desire to take the coronavirus pandemic seriously. He got off to a rocky start almost immediately, tripping up the steps in front of reporters and Pentagon staff members.

“That would have been great, broke my ankle on the way in,” Miller said, according to videos posted on social media.

Miller’s military career was extensive.

Initially an enlisted soldier, Miller became a Special Forces officer, leading Green Beret soldiers in the early days of the wars in Iraq and Afghanistan. He was an honor graduate from the Army’s Ranger School, an Army War College fellow with the CIA and a Special Forces battalion commander during the height of the Iraq War.

“My first experience with Counterterrorism (CT) occurred in 1999 when the Army unit I commanded was tasked to track down and report on Usama bin Laden,” Miller wrote in documents submitted to the Senate for his nomination as National Counterterrorism Center director. “I was a combatant, or provided direct support to fighting elements from 2001 to 2014 as a Special Forces officer.”

Special Forces soldiers who served with Miller described him as a patient and unconventional thinker who earned a reputation for steady leadership.

During the early days of the Afghanistan war, Miller was a company commander in 5th Special Forces Group, partnered with Afghan militias in the fight against al-Qaeda and the Taliban, including future Afghan President Hamid Karzai, according to an Army officer who served with Miller.

Miller was among the first officers in the post-9/11 generation to lose service members in command. Two of his soldiers were killed in an errant U.S. airstrike in December 2001, according to the officer, who spoke on the condition of anonymity because he does not have permission to speak with the news media.

In 2006 and 2007, Miller commanded Special Forces units in and around Baghdad, said Joe Kent, a retired Green Beret who served under him in Iraq and supported Trump’s presidential campaign. Miller’s duties pulled him across large swaths of central Iraq, but he made time to visit Green Berets down to the team level to solicit unvarnished truths about their slice of the war, Kent said.

At the time, some Special Forces leaders were interested in racking up missions and raids. But Miller focused on fusing intelligence with local sectarian dynamics to navigate complex urban battlefields, Kent said.

Miller joined the Trump administration in March 2018, serving through December 2019 as a special assistant to the president and as the senior director for counterterrorism and transnational threats on the National Security Council. In that role, he again made wiping out the remnants of al-Qaeda leadership a top priority.

Javed Ali, who preceded Miller as senior director for counterterrorism at the National Security Council, said that Miller is “not a political operative,” describing him as different from Kash Patel, a former aide to Rep. Devin Nunes (R.-Calif.) and a devout Trump supporter who also served on the NSC staff.

“He’s got years of experience in the military and then the national security policy arena as a professional,” Ali said. “So the hope and expectation for people like me who know Chris is he will not be that person who is going to help achieve political objectives using the department’s tremendous resources and authorities.”

Miller shifted to the Pentagon early this year to be the deputy assistant secretary of defense for special operations, and later began performing the roles of the assistant secretary of defense for special operations, a promotion as vacancies plagued the Trump administration and prompted officials to take on new duties.

Trump nominated Miller in March to be director of the National Counterterrorism Center, and he was confirmed to the position in August. During his confirmation hearing, he told the Senate Select Committee on Intelligence that he still sees himself as a “kid from Iowa who wanted nothing more than to serve his country and make his parents proud.”

Miller said it was his life’s goal, “whether confirmed for this position or in another capacity,” to defeat al-Qaeda and its affiliates and transition America’s war against the group to “a sustainable effort laser-focused on monitoring terrorist threats to the United States.” He described in stark terms the toll that counterterrorism operations have taken on a generation of service members.

“It was not a war we sought, but, in defense of this nation, we selflessly sacrificed our youth and our innocence,” Miller said in his opening statement. “Many dear friends and comrades sacrificed their health, their marriages, and in some cases, their lives. We have no regrets.”

But the Pentagon that Miller will take over has shifted in recent years to focus first on China as its “pacing threat,” as Trump sought to end long-running counterinsurgency wars. While a campaign against China would probably include special operations, it could focus heavily on long-range missiles, hypersonic weapons and other advanced munitions used far from the Middle East.

Sen. Angus King (I-Maine) said in an interview that Esper’s ouster raised concerns on several levels, especially as the United States enters a presidential transition and the Defense Department prepares for it.

“Its essential mission is the security of the country,” King said of the Defense Department. “To fire the leader, particularly during the presidential transition, is a dangerous action which makes no sense.”

King, a member of the Senate Armed Services Committee, questioned why Trump bypassed Deputy Defense Secretary David Norquist and his service secretaries to install Miller. The new acting defense secretary has experience within the department, King said, “but certainly not at the secretary level.”

Rep. Elissa Slotkin (D-Mich.), said that she worked with Miller during her time as a Pentagon official in the Obama administration, and more recently during his time at the National Counterterrorism Center.

“It is critical that he, and all senior Pentagon leaders, remember that they swore an oath to the Constitution, not any one man,” she said. “With that oath comes a commitment to the peaceful transition of power. All leaders must decide what they will do in the next 72 days. I strongly urge Acting Secretary Miller to remember that the country and the military he has dedicated his life to are counting on him to do the right thing.”