In a two-page letter to Mr. Trump obtained by The New York Times, Mr. Esper said, “I serve the country in deference to the Constitution, so I accept your decision to replace me.”
Democrats immediately said Mr. Trump’s removal of Mr. Esper could endanger national security.
“President Trump’s decision to fire Secretary Esper out of spite is not just childish, it’s also reckless,” said Representative Adam Smith, Democrat of Washington and the chairman of the House Armed Services Committee. “It has long been clear that President Trump cares about loyalty above all else, often at the expense of competence, and during a period of presidential transition, competence in government is of the utmost importance.”
Friends and colleagues of the new acting secretary praised Mr. Miller’s Army Special Forces background and counterterrorism credentials but expressed surprise that he had been elevated to such a senior position, even in a temporary capacity. And while he is not considered an ideologue, Mr. Miller does not have the stature to push back on any precipitous actions that Mr. Trump might press in his final weeks in office, colleagues said.
“A move like this probably sends a chill through the senior ranks of the military,” Nicholas J. Rasmussen, a former top counterterrorism official in the Bush and Obama administrations, said in an email. “Not because of anything about Chris Miller personally, though it’s a highly unconventional choice, to be sure. But simply because a move like this contributes to a sense of instability and unstable decision-making at exactly the time when you want to avoid sending that kind of message around the world.”
Mr. Miller is a former Army Green Beret who participated in the liberation of Kandahar early in the war in Afghanistan. He also previously served as the top counterterrorism policy official in the National Security Council in the Trump White House. After that job, he briefly served in a top counterterrorism policy role at the Pentagon this year.
He has been part of the Trump administration’s musical chairs on national security posts. It was only in August that Mr. Miller was named to replace Russ Travers, who was the acting head of the counterterrorism center.
Upon his arrival at the Pentagon on Monday afternoon, Mr. Miller tripped on the stairs and said, “That would have been great, broke my ankle on the way in.”