The ex-Secretary of the US Navy has struck back at Donald Trump for repeatedly involving himself in the case of a Navy SEAL credibly accused by his own brothers-in-arms of war crimes.

Richard Spencer wrote an op-ed in The Washington Post in which he laid out multiple instances in which Trump attempted to intervene in a military review of Chief Petty Officer Edward Gallagher.

"This was a shocking and unprecedented intervention in a low-level review," Spencer wrote.  "It was also a reminder that the president has very little understanding of what it means to be in the military, to fight ethically or to be governed by a uniform set of rules and practices."

Earlier this week, Spencer gave his first TV interview after leaving his post that the reversal of Gallagher's sentence sends the message to troops "that you can get away with things.  We have to have good order and discipline.  It's the backbone of what we do".

Gallagher was acquitted of murdering a teenage prisoner of war in Iraq, but was demoted after being found guilty of posing for photographs with the POW’s body.  Trump restored his rank to chief petty officer earlier this month after reports indicated that a Navy board was considering removing him and kicking him out of the SEALs.  

Spencer also explained decision to attempt to broker a private compromise with the White House on Gallagher by side-stepping the chain of command - the technicality that Trump used to sack him.

"I also began to work without personally consulting Defense Secretary Mark T. Esper on every step," wrote Spencer.  "That was, I see in retrospect, a mistake for which I am solely responsible."

Spencer asked US military partners for their patience and understanding:

"Our allies need to know that we remain a force for good, and to please bear with us as we move through this moment in time," he wrote.