President Donald Trump announced on Monday that he fired defence secretary Mark Esper, making good on his threats to shake up key positions in the final period of his presidency.
“I am pleased to announce that Christopher C Miller, the highly respected director of the National Counter-terrorism Centre, will be acting secretary of defence, effective immediately,” Mr Trump said on Twitter.
“Chris will do a great job. Mark Esper has been terminated. I would like to thank him for his service.”
Mr Trump said Mr Miller's appointment was unanimously confirmed by the Senate.
Mr Esper had prepared a resignation letter in recent days after news that Mr Trump could try to fire him following his loss in the presidential election last week to Joe Biden.
He infuriated the president over the summer by publicly disagreeing with him on using active-duty military troops to quell Black Lives Matter protests and sporadic rioting that erupted in May and June after police killings of unarmed black people.
House armed services committee chairman Adam Smith, a Democrat, immediately condemned Mr Trump’s dismissal of Mr Esper.
“In the national security community, it is well known that periods of presidential transitions leave our country exposed to unique threats,” Mr Smith said.
“Until president-elect Biden is sworn into office next January, it is imperative that the Pentagon remain under stable, experienced leadership.”
“Dismissing politically appointed national security leaders during a transition is a destabilising move that will only embolden our adversaries and put our country at greater risk.
"President Trump’s decision to fire Secretary Esper out of spite is not just childish, it’s also reckless.”
Mr Trump purged other administration officials last week as the prospect of his re-election dimmed.
National Nuclear Security Administration chief Lisa Gordon-Haegarty, who oversees the US nuclear stockpile, offered her resignation on Friday.
Ms Gordon-Haegarty clashed with Energy Secretary Dan Brouillette over the agency’s budget.
Her dismissal drew sharp rebukes even from some of the Trump administration’s closest Republican allies, including the Senate armed services committee chairman, James Inhofe.
“That the Secretary of Energy effectively demanded her resignation during this time of uncertainty demonstrates he doesn’t know what he’s doing in national security matters, and shows a complete lack of respect for the semi-autonomous nature of the National Nuclear Security Administration,” Mr Inhofe said.
The acting administrator of the US Agency for International Development, John Basra, also fired his deputy, Bonnie Glick, on Friday.
The same day, Mr Trump demoted the head of the Federal Energy Regulatory Commission, Neil Chatterjee.