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.
Mr Esper clashed with the president over the White House's use of the military to quell public unrest during protests over racial injustice earlier this year.
In the weeks before Mr Biden takes office on 20 January, Mr Trump is still empowered to make decisions.
