Editor’s Note: Peter Bergen is CNN’s national security analyst, a vice president at New America, and a professor of practice at Arizona State University. His new book is “Trump and His Generals: The Cost of Chaos.” The opinions expressed in this commentary are his own. View more opinion at CNN.

CNN  — 

A dam has broken. Former US Defense Secretary Jim Mattis – who President Donald Trump has compared to one of his heroes, General George Patton – on Wednesday issued a blistering statement saying, “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. We are witnessing the consequences of three years of this deliberate effort.”

Mattis’ statement came after police on Monday attacked peaceful protesters outside the White House with rubber bullets and a noxious gas so Trump could have a photo op with a bible outside St. John’s Church near the White House.

There are three reasons that Mattis’ statement is so important. First, Mattis spent two years working closely with Trump as his first secretary of defense and so he is well-positioned to make the charge that the President has deliberately pursued a policy of division while in office. This is not a charge made by some armchair critic, but by one of the most senior members of Trump’s cabinet who spent untold hours working directly with him.

Second, Mattis is largely revered by the US military and in national security circles for his stellar military record which includes commanding the longest aerial assault in history from an US warship at sea to a landing zone in Afghanistan during the war against the Taliban in 2001. Then Mattis led the US Marines into Baghdad to topple Saddam Hussein in 2003, and he later commanded CENTCOM that oversaw all American wars in the Middle East during the Obama administration.

Get our free weekly newsletter

  • Sign up for CNN Opinion’s newsletter.
  • Join us on Twitter and Facebook

    Third, Mattis has previously gone out of his way not to criticize Trump. He wrote in his 2019 autobiography “Call Sign Chaos,” “I’m old fashioned: I don’t write about sitting Presidents.” And when he was on his book tour promoting his book during multiple interviews, Mattis would not be drawn about his real views about Trump.

    At a book party in Washington, DC, for Mattis that I attended, Mary Louise Kelly, the co-anchor of NPR’s “All Things Considered,” asked Mattis what it would take for him to criticize Trump publicly. Could there ever come a time when he felt he had to speak out if he felt that the country was truly imperiled? Mattis became animated saying he would never do that.

    Obviously Mattis does now feel that the country is imperiled, and he has spoken out in a clear and unambiguous manner that Trump is a threat to the Constitution saying in Wednesday’s statement, “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.”

    Trump, who revels in the ceremonial aspects of being Commander in Chief, now has an emerging bloc of senior retired generals forming against him. Trump will, of course, perform his usual tricks of denigrating them, but he should be aware that if this bloc starts to organize itself to oppose him as he seeks re-election, the President will find it hard to paint these senior generals as a bunch of weak “deep state” types since so many of them performed heroically in the long post-9/11 wars.

    Predictably, only hours after Mattis released his statement, Trump struck back with tweets calling him “the world’s most overrated general” and saying he didn’t like his leadership style.

    Mattis joins a growing anti-Trump chorus of revered retired senior military officers that includes former US Joint Chiefs Chairman Admiral Mike Mullen, who wrote in The Atlantic Tuesday: “It sickened me yesterday to see security personnel … violently clear a path through Lafayette Square to accommodate the president’s visit outside St. John’s Church. I have to date been reticent to speak out on issues surrounding President Trump’s leadership, but we are at an inflection point, and the events of the past few weeks have made it impossible to remain silent.”

    Retired four star Marine General John Allen, who runs the Brookings Institution in Washington, DC, wrote in Foreign Policy magazine Wednesday evening, “To even the casual observer, Monday was awful for the United States and its democracy. The President’s speech was calculated to project his abject and arbitrary power, but he failed to project any of the higher emotions or leadership desperately needed in every quarter of this nation during this dire moment.”

    Trump has now been on the receiving end of multiple blistering critiques by many of the top retired US military generals and admirals. Leading retired four-star officer, Gen. Stanley McChrystal, told ABC News in 2018 that he found Trump to be both immoral and dishonest.

    Adm. William McRaven, the architect of the raid that killed Osama bin Laden, wrote in The Washington Post, in 2018, “Through your actions, you have embarrassed us in the eyes of our children, humiliated us on the world stage and, worst of all, divided us as a nation.”

    Retired three-star Gen. Mark Hertling, a CNN analyst, has also been quite critical of Trump on a number of issues – for instance, when Trump attacked Adm. McRaven for not finding Osama bin Laden sooner, Hertling described this as a “disgusting” attack.

    Even Trump’s current Defense Secretary, Mark Esper, a former officer in the US Army, is distancing himself from Trump, saying publicly on Wednesday that he does not agree with an idea the President has suggested of using the federal military against American protestors who are exerting their First Amendment rights.

    Esper has now made the cardinal Trumpian sin of telling the truth in public about the Great Leader, so we should not be surprised if he is eventually booted from Trumpworld to be replaced with someone more compliant.