Trump this month launched a sweeping shakeup of the US Defense Department, firing top officers and moving to lay off thousands of civilian workers, pushing the Pentagon into the political spotlight.

Democrats have accused Trump and Defense Secretary Pete Hegseth of seeking to politicize the military and ensure it is led by people personally loyal to the president.

In the letter, the ex-defense secretaries, who served under both Republican and Democratic administrations, added their voices to the condemnation, saying they were "deeply alarmed" by the firings.

"We write to urge the U.S. Congress to hold Mr. Trump to account for these reckless actions and to exercise fully its Constitutional oversight responsibilities," the letter said.

The letter was signed by the most recent defense chief under Trump's Democratic predecessor Joe Biden, Lloyd Austin, as well as William Perry, Chuck Hagel, Leon Panetta and James Mattis, who served in Trump's first administration.

"Mr. Trump's dismissals raise troubling questions about the administration's desire to politicize the military and to remove legal constraints on the President's power," the five men wrote.

"President Trump's actions undermine our all-volunteer force and weaken our national security."

Remaining above the political fray is a core principle for the US armed forces, with troops even barred from engaging in some types of political activity to maintain the military's neutrality.

Trump administration officials have defended the firings, with Hegseth insisting the president is simply choosing the leaders he wants, saying, "Nothing about this is unprecedented."

No accusations of misconduct have been made against the recently removed officials, including top officer General Charles "CQ" Brown, who was less than two years into his four-year term as chairman of the Joint Chiefs of Staff.

Since taking office in late January for his second term, Trump has instigated rapid, broad cuts to US federal agencies.

The Defense Department -- the largest employer in the United States -- said last week it would soon start cutting its civilian workforce by at least five percent.