Five former defense secretaries issued a scathing letter on Thursday assailing President Donald Trump for firing the chairman of the Joint Chiefs of Staff and other senior officers. 

In an open letter published Thursday, former Defense Secretaries James Mattis, Leon Panetta, Lloyd Austin, Chuck Hagel, and William Perry urged Congress to hold immediate hearings on Trump’s recent firings of Joint Chiefs Chairman Gen. CQ Brown and several other senior military leaders. 

They said senators should ‘refuse to confirm’ any new DOD nominations in response to the firings.

In the letter, they alleged that Trump’s actions undermined ‘our all-volunteer force and weaken our national security’ and they accused the president of trying to turn the apolitical U.S. military into an instrument of partisan politics and using firings, which extend to the top Army, Navy and Air Force lawyers, to do so.

All defense secretaries but one, James Mattis, served under Democratic administrations. 

Trump announced the firings late on Friday, but his administration has yet to clarify in any detail what caused the unprecedented shakeup, which also included the dismissal of the head of the Navy, Admiral Lisa Franchetti, the first female officer to lead a military service.

Air Force General C.Q. Brown was only the second Black officer to become Joint Chiefs chairman and he was less than halfway through his four-year term when he was let go.

‘Mr. Trump’s dismissals raise troubling questions about the administration’s desire to politicize the military,’ they wrote. ‘We, like many Americans – including many troops – are therefore left to conclude that these leaders are being fired for purely partisan reasons.’

Fox News Digital has reached out to the White House for comment on the letter. 

The former defense secretaries called on Congress to hold hearings to ‘assess the national security implications’ of Trump’s dismissals. Republicans hold a majority in both chambers.

The letter cautioned that the actions at the Pentagon could deter Americans from choosing a life in the military, should their careers be judged through the lens of partisan politics. It could also have a chilling effect on speaking ‘truth to power,’ they cautioned.

‘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,’ they wrote.

Fox News’ Liz Friden and Reuters contributed to this report.