WASHINGTON — President Donald Trump announced the ouster of Defense Secretary Mark Esper on Monday, marking the exit of Trump's fourth Pentagon chief and naming a senior intelligence official to take his place.

"Mark Esper has been terminated. I would like to thank him for his service," the president said on Twitter.

Trump said that Christopher Miller, director of the National Counterterrorism Center, would immediately become acting defense secretary. "Chris will do a GREAT job!" Trump wrote.

Esper's firing, expected since at least June but announced suddenly as Trump continues to contest the results of the presidential election, plunges the Pentagon into another period of leadership upheaval as it navigates an unusual transition period fraught with political tensions and potential security risks.

An Army veteran and former weapons lobbyist who was confirmed as defense secretary in July 2019, Esper was mostly aligned with his commander in chief on major foreign policy issues, but clashed with Trump over the president's steps to draw the military into partisan politics.

Chief among those was a dust-up in June, when Trump demanded that thousands of troops be dispatched on the streets of Washington amid protests over the police killing of George Floyd. Trump threatened to invoke the Insurrection Act to active-duty service members, but Esper and Gen. Mark Milley, chairman of the Joint Chiefs of Staff, pushed back, concerned it would look like martial law.

Two officials, who spoke on condition of anonymity because they were not authorized to comment on the record, said that Trump's reaction to Esper's public opposition to using troops in June was as heated and fierce as they'd ever seen from the president.

The president had to be talked out of firing Esper that week, and, throughout the fall and campaign season, aides said he regularly raising firing Esper. The president believed Esper was trying to embarrass him and damage his re-election prospects, the officials said. In recent months, Esper has barely seen the president.

Esper in the meantime has presided over the military's response to the coronavirus pandemic, sought to take steps to address racial and gender discrimination, and announced troop cuts in line with Trump's foreign policy goals. But he also continued to defy the president on other matters that Pentagon officials see as threatening the military's status as one of the nation's respected nonpartisan institutions.

In July, Esper issued a de facto ban on the display of the Confederate battle flag on military bases and stated openness to renaming Army posts that recognize Confederate officers who fought to preserve slavery. Trump angrily tweeted that he would not allow bases to be renamed but did not overturn the ban.

Esper took over amid earlier upheaval in 2019, after then-acting Defense Secretary Patrick Shanahan abruptly resigned in the midst of preparations for his confirmation hearing. Shanahan served in an acting capacity for six months after his predecessor, retired Gen. Jim Mattis, resigned over disputes with Trump about military alliances and other matters. Richard Spencer, the then-Navy secretary, served as acting defense secretary for a week while Esper underwent Senate confirmation.

"The irony is that the president derided him as 'Mark Yesper' and the reason that he got fired is that he wasn't," said Eric Edelman, a former top Pentagon official under George W. Bush. "It's mean spirited vengeance and vindictiveness, and he's doing it because he can."

But Esper has also struggled at times to achieve his stated goal of protecting the military from politicization.

In the same week he broke with Trump in June over the use of active duty troops to restore order, he referred to U.S. cities as a "battlespace," a remark for which he apologized. He also drew criticism for appearing alongside Trump for a photo outside the White House shortly after uniformed personnel cleared protesters from the area.

It was not immediately clear whether the Trump administration would seek Senate confirmation for Miller, an Army Special Forces veteran with more than 30 years of government service, before President-elect Joe Biden takes office in January.

Miller previously oversaw Special Operations policy at the Pentagon and served as a top counterterrorism official at the White House National Security Council, where he focused on pressuring the Islamic State, hostage recovery and hunting down the remnants of al-Qaeda's leadership. On his watch, the U.S. military killed Islamic State leader Abu Bakr al-Baghdadi and a key lieutenant.

A respected, hard-charging official, Miller was part of the first deployment of U.S. special operations troops to Afghanistan in 2001 and to Iraq in 2003.

Arnold Punaro, a retired Marine Corps general with extensive experience preparing nominees for Senate confirmation, said he did not expect Miller's appointment to present no legal issues, since Miller has has already undergone Senate confirmation prior to being sworn in as CTC director in August.

But some analysts and politicians warned the abrupt shuffle at the top of the Pentagon was ill-timed and dangerous, citing geopolitical crises that have required the engagement of top national security officials during past presidential transitions.

The Iranian Hostage Crisis continued to play out during the transition from President Jimmy Carter to President Ronald Reagan in 1980. The bombing of Pan Am Flight 103 over Lockerbie, Scotland, occurred during the transition from Reagan to President George H.W. Bush in 1988. Four years later, Bush ordered U.S. troops into Somalia as part of a United Nations mission during the transition from Bush to Clinton in 1992.

Rep. Adam Smith, D-Wash., the chairman of the House Armed Services Committee,called the firing a destabilizing move that will embolden adversaries and puts the United States at greater risk.

"Until President-Elect Biden is sworn into office next January, it is imperative that the Pentagon remain under stable, experienced leadership." Smith said. "It has long been clear that President Trump cares about loyalty above all else, often at the expense of competence, and during a period of presidential transition competence in government is of the utmost importance."

One Republican official who works in national security, speaking on the condition of anonymity due to the sensitivity of the issue, called Trump's decision "irresponsible and petty."A steady handoff of national security responsibilities is a basic expectation, the official said.

Mark Cancian, a former Marine officer and White House budget official, said a possible confirmation effort could prove an uphill battle for Miller, while a respected intelligence official was serving in a mid-level Pentagon role only several month ago.

"He doesn't have that national reputation" Cancian said.

The Washington Post's Ellen Nakashima and Alex Horton contributed to this report.