President Donald Trump fired Defense Secretary Mark Esper on Monday, an unprecedented move by a president struggling to accept election defeat and angry at a Pentagon leader he believes wasn’t loyal enough.

The decision, which could unsettle international allies and Pentagon leadership, injects another element of uncertainty to a rocky transition period as Joe Biden prepares to assume the presidency.

Presidents who win reelection often replace Cabinet members, including the secretary of defense, but losing presidents have kept their Pentagon chiefs in place until Inauguration Day to preserve stability in the name of national security.

Trump announced the news in a tweet, saying that “effective immediately” Christopher Miller, the director of the National Counterterrorism Center, will serve as acting secretary, sidestepping the department’s No.2-ranking official, Deputy Defense Secretary David Norquist.

“Chris will do a GREAT job!” Trump tweeted. “Mark Esper has been terminated. I would like to thank him for his service.”