President Donald Trump has sacked Defence Secretary Mark Esper, announcing on Twitter that the top US official has been “terminated”.
Christopher Miller, the current head of the National Counterterrorism Center, will take on the role immediately.
It follows a public falling-out between Mr Trump and Mr Esper in recent weeks.
Mr Trump has so far not conceded the US election to President-elect Joe Biden, and has vowed to challenge the projected result in court.
In the weeks before Mr Biden takes office on 20 January, Mr Trump is still empowered to make decisions.
Mr Miller was seen entering the Department of Defense headquarters at the Pentagon on Monday shortly after Mr Trump announced the dismissal.
The former Special Forces soldier served on President Trump’s National Security Council before becoming head of the Counterterrorism Centre in August.
In a resignation letter, Mr Esper thanked members of the Armed Forces and said he was proud of his achievements in 18 months of service at the Pentagon.
“I serve my country in deference to the Constitution, so I accept your decision to replace me,” he wrote.
Top Democrat Nancy Pelosi criticised the decision. “The abrupt firing of Secretary Esper is disturbing evidence that President Trump is intent on using his final days in office to sow chaos in our American Democracy and around the world,” said the speaker of the House of Representatives.