President Donald Trump has fired his defense secretary Mark Esper following rumours the Pentagon chief was preparing to step down.

In a pair of tweets, Trump announced that Christopher Miller, the director of the National Counterterrorism Center, will serve as acting defense secretary.

“Mark Esper has been terminated,” the president tweeted. “I would like to thank him for his service.”

Esper had prepared a resignation letter before the election results were final because he was one of the cabinet members expected to be ousted should Trump win, NBC News reported last week.

Trump has reportedly been unhappy with Esper’s performance for months.

NBC News reported in September that White House officials approached veterans affairs secretary Robert Wilkie about potentially taking the top spot at the Pentagon should Trump decide to fire Esper.

Esper, who has headed the Defense Department since June 2019, has broken with Trump on several occasions. In June, Esper called it “a mistake” to have accompanied the president days earlier in his infamous photo op in front of a church near the White House amid anti-racism protests. He also defied Trump’s threat to use active-duty troops to quell the nationwide protests.

A month later, Esper reportedly drew Trump’s ire by effectively barring Confederate flags on military bases.