This week’s warning by all 10 living former U.S. defense secretaries that the military must not be used to determine an election is a stunning pushback against President Donald Trump’s drive to retain power.

The fact that this disparate group of Democrats and Republicans, including Donald Rumsfeld, Dick Cheney, and ex-Trump Pentagon chiefs James Mattis and Mark Esper, felt the need to speak out together illustrates the danger of this moment.

As the 10 made clear in an op-ed for the Washington Post, the threats to the constitution by Trump and his GOP allies in Congress are so grave that no one knows whether he’ll try to drag the armed forces into overturning the election.

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“As former secretaries of defense … each of us swore an oath to support and defend the Constitution,” the group wrote. “We did not swear it to an individual or party … Efforts to involve the U.S. armed forces in resolving election disputes would take us into dangerous, unlawful and unconstitutional territory.” Indeed.

I asked Eric Edelman, undersecretary of defense in the George W. Bush administration, what exactly prompted the statement.

It began with a conversation between him and Dick Cheney, he said, “and then it just snowballed. We got all 10 within 48 hours over New Year’s Eve and Day. Though they may not agree politically on anything, when it comes to the norms and institutions of democracy, they all agree.”

Their agreement was based on concern over what once seemed unthinkable. “We were all concerned about Trump’s unpredictability, afraid he might do something that would drag the military in. We wanted to reinforce [Chairman of the Joint Chiefs of Staff Gen. Mark] Milley’s position that the military has no part in elections.”

This was especially true, Edelman said, after Trump’s disgraced (and pardoned) ex-national security adviser Michael Flynn visited the Oval Office. The visit came right after his outrageous Dec. 17 interview with the pro-Trump Newsmax media, in which Flynn said the president “could take military capabilities and place them in those [swing] states and basically rerun an election.” The president no doubt loved the interview.

“It was a whole pattern of behavior” that worried the 10 men, Edelman continued — including rash moves by the new partisan Trump appointees to top Pentagon posts. “Everyone was concerned Trump might invoke the Insurrection Act,” which would allow him to mobilize active duty military, using the excuse of street violence by pro-Trump groups that the president is urging to protest the election.

There was also concern the president might provoke an international crisis — perhaps with Iran — to try to retain power.

“We didn’t know about the tape at the time of the statement,” Edelman said, referring to the tape of Trump’s outrageous call to Georgia’s secretary of state, demanding that he “find” enough votes to turn the president’s defeat into victory. “But it shows how he thinks. He is willing to do anything to stay in office. That justifies concern.”

What’s so stunning about this group statement is that it takes seriously a possibility that most Americans, myself included, dismissed until now — that Trump might actually try to pull a military coup.

The idea still appears highly dubious. The president only has two weeks left in office, and even his cowardly GOP claque in Congress might desert him. Moreover, the military would no doubt slow walk any such unconstitutional effort until Jan. 20.

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Yet, the president’s patsies at the Pentagon, including Acting Defense Secretary Christopher Miller, might be willing to issue his orders. “You have a situation where DOD has been pressed by the president to politicize itself,” says former Undersecretary of Defense Dov Zakheim. “Trump seems to think DOD is an arm of the Oval Office to defend anything he does. And nobody knows what he might do.”

What also inspired this extraordinary statement was the president’s unconstitutional attack on the elections and the spinelessness of GOP legislators who abet him. Even more dangerous, the endless stream of lies about election fraud Trump pours forth on Twitter and at rallies, amplified by pro-Trump media (Newsmax, OAN, Fox commentators), has convinced a large part of the country that elections can’t be trusted.

Even if he has to be carried out of the White House and can no longer command troops, Trump will no doubt keep spreading his poison, agitating for civil war. But until he’s gone — and even after — the number and heft of serious political leaders who challenge his lies is critical. And that number will be vital to helping President-elect Joe Biden get any business done.

So the defense secretaries’ statement is truly significant. “It is a historical document, completely unprecedented,” says the Brookings Institution’s Thomas Wright. “It’s a shot across the bow. People will look back and say this is what was necessary. If Trump doesn’t succeed, it will be because there was pushback. That is probably why the former secretaries thought they shouldn’t wait, should act now.”

And should continue to speak out.