We do not yet know precisely why Defense Secretary Mark T. Esper publicly broke with President Trump on Wednesday, renouncing the use of the Insurrection Act as a means to deploy the military against civilian demonstrators. We can surmise, however, that Pentagon brass was finally fed up and prevailed upon Esper to speak out.

Chairman of the Joint Chiefs of Staff Gen. Mark Milley, who had accompanied Trump on his march across Lafayette Square, put out a memo on June 2, which read like a not-very-subtle rebuke of Trump’s attempt to use the military to suppress protesters:

1. Every member of the U.S. military swears an oath to support and defend the Constitution and the values embedded within it. ... We in uniform — all branches, all components, and all ranks — remain committed to our national values and principles embedded in the Constitution.
2. During this current crisis, the National Guard is operating under the authority of state governors to protect lives and property, preserve peace, and ensure public safety.
3. As members of the Joint Force-comprised of all races, colors, and creeds — you embody the ideals of our Constitution. Please remind all of our troops and leaders that we will uphold the values of our nation, and operate consistent with national laws and our own high standards of conduct at all times.

James N. Miller, a former undersecretary of defense for policy, announced his resignation from the Science Defense Board in the pages of The Post and upbraided Esper:

As a concerned citizen, and as a former senior defense official who cares deeply about the military, I urge you to consider closely both your future actions and your future words. For example, some could interpret literally your suggestion to the nation’s governors Monday that they need to “dominate the battlespace.” I cannot believe that you see the United States as a “battlespace,” or that you believe our citizens must be “dominated.” Such language sends an extremely dangerous signal.

Gen. David Goldfein, the Air Force chief of staff, and Lt. Gen. Jay Silveria, superintendent of the Air Force Academy, also spoke up this week in support of the protests for racial justice, with Silveria directly repudiating use of violence against fellow Americans.

In addition, Air Force Gen. Joseph L. Lengyel, who heads the National Guard Bureau, put out a statement Wednesday entitled “We Must Do Better,” denouncing the racism that has resulted in the deaths of so many unarmed African Americans, urging Americans to listen and learn and reminding us, “Everyone who wears the uniform of our country takes an oath to uphold the Constitution and everything for which it stands.” He declared that if they are to uphold their oath as service personnel and “decent human beings” they must uphold the oath.

But the biggest voice, with the widest reach, to step into the fray did so in one of the most stunning repudiations of a sitting president by a former Cabinet official — let alone a former general. Former defense secretary Jim Mattis blasted Trump for dividing America and accused him of unconstitutional action.

“Donald Trump is the first president in my lifetime who does not try to unite the American people — does not even pretend to try. Instead, he tries to divide us,” Mattis wrote on Wednesday in the Atlantic. “We are witnessing the consequences of three years of this deliberate effort. We are witnessing the consequences of three years without mature leadership.”

He continued, "When I joined the military, some 50 years ago, I swore an oath to support and defend the Constitution. Never did I dream that troops taking that same oath would be ordered under any circumstance to violate the Constitutional rights of their fellow citizens — much less to provide a bizarre photo op for the elected commander-in-chief, with military leadership standing alongside.”

Mattis’s unprecedented rebuke raises a number of issues.

First, he was widely and justifiably criticized for failing to speak out previously against Trump, not even to share direct observations that might persuade lawmakers and Americans that Trump is unfit for office. That failure remains, and we do not know whether speaking up earlier would have deterred Trump from further action. Nevertheless, no one should diminish the importance of his action, which may carry sway with other current military officials, Congress and the public. It is late, but it better than anything we have heard from any other former administration official. (Contrast Mattis’s action with the refusal of former national security adviser John Bolton, who chose to hold back direct knowledge of Trump’s alleged impeachable conduct for the sake of a book deal.)

Second, it remains unclear whether Mattis will hold any sway with Republican lackeys in the Senate who refuse to break with Trump — or worse, who try, as Sen. Tom Cotton (R-Ark.) shamefully did, to outdo Trump in vowing to use the military against civilians. Most of them long ago tied themselves to Trump’s mast, willing to go down with him — and take the country with them — rather than be on the receiving end of a Trump Twitter tirade. These lawmakers are political cowards more likely to rationalize Mattis’s statement as sour grapes rather than take the warning to heart. The throng of Republican political lackeys is unlikely to have a political epiphany. They must be defeated at the ballot box.

Third, together with the extraordinary written statement from former president George W. Bush, one wonders whether we are on the verge of a sort of popular-front moment when a significant faction of the right joins with Democrats to defeat an authoritarian incumbent. If Mattis and Bush can call out Trump’s egregious, unconstitutional and anti-American conduct, couldn’t they endorse former vice president Joe Biden, the presumptive Democratic nominee?

Finally, Trump may either retreat from a direct clash with the military (just as he retreated to the bunker when the protests heated up), or he may insist on using the military on Americans. We are at a uniquely precarious time in U.S. history, one in which the American people will need to rely on the decency and patriotism of those who have sworn an oath to defend and protect the Constitution. It’s a time for choosing for military officials, for politicians, for law enforcement and, most of all, for all Americans of good will.

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