Donald Trump sacked Defense Secretary Mark Esper on Monday, in his first major act since losing re-election.
"Mark Esper has been fired," wrote Trump on Twitter. Christopher Miller, a senior intelligence official, will take over as acting defense minister "with immediate effect," he added.
Mr Trump fell out with his Secretary of Defense in June after Mr Esper said he was against invoking the Insurrection Act to send U.S. troops onto American roads to quell civil unrest, except as a last resort amid a host of protests against racism.
Mr Trump had publicly ridiculed Mr Esper as "Yesper," a nickname of colleagues who had previously seen him as a yes man who failed to assert himself against Mr Trump.
Mr. Esper also supported a bipartisan Congressional effort to rename military bases that were named after former Confederate leaders, some of whom owned slaves. Mr Trump vowed to veto the annual defense spending bill if it contained such provisions.
Resignations are common among senior political figures in outgoing administrations, but the announcement of Mr Esper's dismissal on Twitter was particularly unusual as Mr Trump has yet to admit defeat in the presidential election – and there are still two months to go before the Biden administration takes over Office.
"The dismissal of a competent defense minister with two months remaining is exactly the kind of petty recklessness that led the Republican defense company to endorse Joe Biden as president," said Kori Schake, foreign policy and defense expert for the conservative American Enterprise Institute .
James Stavridis, a retired admiral and former US commander of the European and Southern Forces, argued the move "made no sense at this point".
"Things are already unstable internationally and that doesn't help," he said on Twitter, hoping opponents wouldn't try to take advantage of it.
Mr. Miller, a former Green Beret, is currently the director of the National Counter Terrorism Center, a unit within US intelligence. He is a veteran of the conflicts in Afghanistan and Iraq.
During Mr. Trump's tenure, he served first as a counter-terrorism officer on the National Security Council and then as a senior special operations officer in the Pentagon. Mr. Miller was sworn in to his final role in a controversial move in the office of the Director of National Intelligence in August.
A former colleague in the intelligence community described Mr. Miller to the Financial Times as "quirky" and "a good guy with a great temper." Another described him as "a professional" and "a known crowd".
The Pentagon declined to comment and referred inquiries to the White House.
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