One defense official described the situation to CNN as "scary" and "very unsettling."
The president's motives for the Pentagon shake-up are unclear, but it has created a great deal of unease within the building.
Three Trump allies stepped into new roles at the Defense Department, officials announced.
Democrats on Capitol Hill are calling Esper's ouster with two months remaining in Trump's administration reckless.
Acting Under Secretary of Defense for Policy James H. Anderson has submitted his resignation.
Miller is not among the most prominent national security names.
President Donald Trump has fired Defense Secretary Esper days after news outlets projected Trump would not win reelection....
Esper called his brief tenure as leader of the Pentagon "the honor and privilege of a lifetime."
There's a consensus in Washington, D.C. that Defense Secretary Mark Esper is on thin ice with President Donald Trump.
It is a critical time in the U.S.-India relationship
The Heritage Index assessed U.S. military strength across the service branches on a five-tiered scale.
U.S. Army Recruiting Command is reducing its force by roughly 1,700 recruiters over the next year.
Robert Teichgraeber, 100, survived 421 days as a prisoner of war.
By the end of the spring, there will be 294 new naval aviation officers.
Members of II Marine Expeditionary Force wrapped up a training exercise last week that ran from North Carolina to New York.