Trump announced his dismissal of Defense Secretary Mark Esper on Twitter on Monday, signaling he may use his final months in office to settle scores within his administration.
But the further reshuffling at the Pentagon is raising concern among Democrats about whether U.S. national security policy may become unsettled as Republican Trump exits office.
It could also potentially make it easier for Trump to execute policies that Esper had opposed, such as deploying active duty troops to suppress street protests in the United States.
Trump's move sent a dangerous message to America's adversaries and dimmed hopes for an orderly transition as Democratic President-elect Joe Biden prepares to take office, Democrats said.
"It is hard to overstate just how dangerous high-level turnover at the Department of Defense is during a period of presidential transition," said Representative Adam Smith, the Democrat who leads the House Armed Services Committee.
Esper was replaced by Christopher Miller, who had been the director of the National Counterterrorism Center. The Pentagon said Kash Patel, who was the top counter-terrorism adviser on the White House National Security Council, would be Miller's chief of staff.
Patel worked as a top aide to Representative Devin Nunes, the pro-Trump Republican who chaired the House Intelligence Committee and now is its top minority member. While working for Nunes, Patel helped produce a memo accusing the FBI and Department of Justice of bias against Trump.
In the wake of Esper's departure, the Pentagon's top policy adviser resigned, allowing that post to be filled by Anthony Tata, a retired Army brigadier general who has called Obama "a terrorist leader."
Tata failed to secure a Senate confirmation hearing in August and was performing the duties of the deputy undersecretary of defense for policy.
The reshuffle raises the possibility that Trump will try to make good on still unfulfilled campaign pledges before Jan. 20, when Biden takes office. Those include potentially ordering a full withdrawal of U.S. forces from Afghanistan.Representative Elissa Slotkin, a Democrat who served as a senior Pentagon official in the Obama administration, called on Miller to put national security interests ahead of loyalty to Trump, saying "the country and the military he has dedicated his life to are counting on him to do the right thing."