Eight former defense secretaries and five ex-joint chiefs chairmen signed a statement on 16 "Best Practices of Civil-Military Relations" that came after several years -- particularly under former president Donald Trump -- in which the Pentagon became enmeshed in political machinations.

"We are in an exceptionally challenging civil-military environment," they wrote.

"Politically, military professionals confront an extremely adverse environment characterized by the divisiveness of affective polarization that culminated in the first election in over a century when the peaceful transfer of political power was disrupted and in doubt," they said.

"Looking ahead, all of these factors could well get worse before they get better."

The statement, published by the defense-focused "War on the Rocks" website, did not cite any examples to illustrate civil-military tensions.

But it did make reference to the challenge to the 2020 election results by Trump and his supporters that led to the violent January 6, 2021 assault on the US Capitol.

The Pentagon has been accused of stalling the deployment of National Guard troops to confront the attackers.

Also during the Trump years, military personnel were asked to help in a number of non-traditional activities, including building a border wall and guarding the border against undocumented migrants, and helping police cities hit by violent protests.

- Legal orders -

In one incident, Trump had then-defense secretary Mark Esper and General Mark Milley, who is still the chairman of the Joint Chiefs of Staff, walk alongside him in front of the White House after police cleared the street of people protesting the murder of Black man George Floyd by police.

Both later expressed regrets they took part in what was widely labelled a political "photo op" for the president.

Under President Joe Biden, the military has been forced to undertake a haphazard and deadly withdrawal from Afghanistan that senior Pentagon leaders did not agree with.

And Biden was widely criticized last week for giving a deeply political speech attacking Trump's supporters while two Marine guards stood behind him.

The officials stressed that the military leadership must accept orders even when they disagree with them, but said the orders must be legal.

"Regardless of the process, it is the responsibility of senior military and civilian leaders to ensure that any order they receive from the president is legal."

The statement was signed by defense chiefs under both Democratic and Republican administrations, including Robert Gates, Leon Panetta, Mark Esper and James Mattis. The latter two served under Trump and were both fired after they clashed with the president.