The Trump administration launched a dramatic shake-up at the senior levels of the Pentagon on Tuesday, installing three White House loyalists in influential roles and intensifying turmoil a day after President Trump abruptly fired his defense secretary.

The changes, confirmed by the Pentagon in a statement Tuesday evening, have alarmed Democrats and some Republicans and promised to complicate a transition to a Biden administration. On Monday, Trump took to Twitter to announce the ouster of his fourth defense secretary, Mark T. Esper, upending decision-making in the Pentagon as the president refuses to concede the election he lost last week to former vice president Joe Biden.

In addition to Christopher C. Miller, who was chosen Monday as the new acting defense secretary, the officials in new positions are Kash Patel, a Trump loyalist and former aide to Rep. Devin Nunes (R-Calif.), and Anthony J. Tata, whose nomination to be undersecretary of defense for policy crumbled over the summer amid scrutiny by Democratic lawmakers of his tweets and remarks promoting conspiracy theories and calling former president Barack Obama a terrorist.

President Trump fired Defense Secretary Mark T. Esper in a Nov. 9 tweet, marking the fourth Pentagon chief the president has let go during his administration. (Reuters)

Tata, who had served as an adviser to Esper, will now perform the duties of the undersecretary for policy, following the resignation Tuesday of James Anderson, a former Marine intelligence officer who had served in the Trump administration since 2018.

Also ousted at the Pentagon is Joseph Kernan, a retired three-star admiral and Navy SEAL officer who had served as undersecretary of defense for intelligence. He will be replaced by Ezra Cohen-Watnick, 34, who most recently served as the acting assistant secretary of defense for special operations and low-intensity conflict. He first joined the Trump administration as a National Security Council director under former national security adviser Michael Flynn.

The decisions swept decades of experience out of the Pentagon as the Trump administration seeks to eject people from government whom it does not view as sufficiently loyal to the president. The Defense Department, with its mandate to remain loyal to the U.S. Constitution and keep uniformed personnel at a distance from partisan politics, could see more departures in coming days.

An administration official said late Tuesday that Trump remains determined to withdraw troops around the world before leaving office.

“He sees the Pentagon as the leader of the resistance to his agenda,” said the official, who like several others interviewed for this report spoke on the condition of anonymity because of the sensitivity of the issue.

Another administration official said the moves were requested by the White House and constituted a Pentagon takeover by Trump’s National Security Council staff. They coincide with ongoing debates about the pace of troop withdrawals from several countries, including Afghanistan and Somalia.

The changes come after months of tension between not only the president and Esper, but also national security adviser Robert C. O’Brien and Esper, two administration officials said.

“POTUS never appreciated that the military wasn’t political, and still doesn’t,” one senior White House official said. “Most of the fights Esper had with him were about the politicization of the military.”

Trump has long chafed at military leaders who have resisted his entreaties to make available military weaponry, including tanks and jets, for Trump-orchestrated holiday displays.

He also grew angry when Esper and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, walked with him in Lafayette Square in June for a photo opportunity in front of a church and then expressed regret in the incident later, saying they should not have participated. Officials close to Esper and Milley said they were unaware that they were going to be used as they were that day, and Trump seethed for days and considered firing Esper then.

The president has told staff members repeatedly that most of the military is Republican and will support him, the White House official said, though studies and polling show that the military is split politically.

A senior defense official said that Pentagon staff members were told on Monday, before Esper was fired, that transition questions should be routed to his chief of staff.

The housecleaning triggered a new round of condemnations from Democrats, who already were alarmed by Trump’s unwillingness to concede the election.

“It is hard to overstate just how dangerous high-level turnover at the Department of Defense is during a period of presidential transition,” said Rep. Adam Smith (D-Wash.), chairman of the House Armed Services Committee. He called the departure of Anderson “something that should alarm all Americans” and highlighted that Tata’s confirmation process was derailed by his controversial comments.

Miller, who completed his first full day in his new job on Tuesday, has not addressed the news media since assuming the role. Assigned to a deputy assistant secretary of defense position, he has ascended rapidly, becoming an acting assistant secretary of defense and then director of the National Counterterrorism Center in August, and bypassing numerous other senior Pentagon officials to become acting defense secretary. Typically, the deputy defense secretary or a service secretary would becoming the acting Pentagon chief.

A former administration official said Miller has been considered as a possible replacement for Esper for weeks. Patel, who aggressively defended Trump during his impeachment, worked for Miller on the National Security Council and is seen by him as a trusted hand, the official said.

Miller, a counterterrorism expert, became close to O’Brien while O’Brien was serving as Trump’s hostage negotiator, the official said. John McEntee, who has sought to stamp out perceived disloyalty in the Trump administration as director of presidential personnel, also supports Miller, the official added.

The personnel changes come as the Trump administration blocks efforts by the Biden team to begin the transition process, including in matters of national security. Three defense officials, speaking on the condition of anonymity because of the sensitivity of the issue, said the Defense Department is following a legal requirement stating that a presidential transition cannot begin until it receives “ascertainment” by the administrator of the General Services Administration.

That GSA official, Emily Murphy, has declined to issue such a notification.

“Once it does occur, and should there be a President-Elect, federal resources will become available to the President-Elect Transition Team by statute,” a defense official said in an unsigned statement released by the Defense Department. “DOD has had no contact with transition teams from either campaign, and will not until notified of ascertainment by the GSA Administrator.”

Secretary of State Mike Pompeo took it farther on Tuesday, possibly joking during a news conference.

“There will be a smooth transition to a second Trump administration,” he said.