We’ve gotten used to so much in Donald Trump’s two years in office, from the cruelty of his immigration policies to his childlike understanding of international trade, and from his apparent fear of Vladimir Putin to his whipsawing of the financial markets. Too many Americans have simply become accustomed to the president’s antics as a normal part of the background noise of their lives. Sometimes Trump’s disjointed thoughts are merely an embarrassment; at other times, he makes dangerous policy changes or wipes out great amounts of wealth in a tweet.

Now, however, the president has opened a Pandora’s box by escalating his attacks on senior U.S. military leaders. No American president has ever dared risk the American civil-military relationship for less cause or with such childish malice.

American citizens don’t think much about civil-military relations. That’s good: It’s a sign of a healthy polity. We don’t worry about tanks rolling up North Capitol Street or paratroopers lolling about in Lafayette Square. We’re not that kind of country. But the president has taken a dangerous path, excoriating retired military leaders who criticize him and lavishing praise and make-believe pay raises on the active-duty military voters who he believes support him. A precious heritage built on the dual pillars of military obedience to civilians and civilian respect for military professionals is now at severe risk.

The president began New Year’s Day with a blast at retired Army General Stanley McChrystal, who in a recent interview indicated that he would never be able to join a Trump administration because he thinks the president is “immoral.” Trump, resorting to the limited vocabulary that has marked his later years, tweeted that President Barack Obama had fired McChrystal “like a dog” and that McChrystal is known for his “big, dumb mouth,” and that he is a “Hillary lover!” (The exclamation point, of course, is de rigueur.)

Read: A former general’s new broadside against President Trump

This wasn’t Trump’s first shelling of senior military leaders. In December 2018, he referred to unnamed critics of his slapdash decision to pull U.S. forces from Syria as “failed generals.” During the 2016 campaign, he lashed out at Colin Powell and John Allen, Obama and Hillary Clinton supporters, respectively, not for their politics but specifically to ridicule their abilities as commanders.

Once elected, Trump hired a number of well-known generals, not only to assuage the fears of his own party, but in order to bask in the reflected glow of their stars. He put John Kelly at Homeland Security and, in a departure from tradition, sought a waiver from Congress to bring recently retired Marine General James Mattis into civilian government sooner than the law allowed. Congress, relieved to have a man of Mattis’s character and experience between Trump and the Pentagon, granted the request.

Of course, he also put retired Army General Michael Flynn—now a convicted felon—in the national-security adviser’s chair. When Flynn had to be forced out, Trump reached out to an active-duty officer, H. R. McMaster, who could hardly say no to a direct request from the president of the United States. By the summer of 2017, generals headed the National Security Council, the Pentagon, the White House staff (once Kelly replaced the hapless Reince Priebus), and even the U.S. prison system.

This, in itself, was a bad idea. Retired generals and admirals are invaluable national resources who should be called back into government service sparingly and only for the most important reasons. Even when they retire, they can never really take their stars off, and placing them in positions of senior civilian leadership should be done with great caution. It should not be done purely to spackle the cracks in a president’s ego, or as a shot of political Xanax to calm the fears of a jittery nation. It is the most anti-republican of sentiments to have Americans reassuring one another, as they have for two years, that all is well because the generals are really in charge.

Read: The president is visiting troops in Iraq. To what end?

Worse, it is fundamentally anti-American to disparage the courage and ability of senior military leaders merely because they are exercising their First Amendment rights on return to civilian life. Trump’s shots at everyone from John McCain to McChrystal should be regarded by any person of goodwill as despicable, but that is a personal judgment each of us must reach on our own.

From a political standpoint, however, what the president is doing is corrosively dangerous. He is impugning the character and competence of senior U.S. military leaders purely for political reasons. He is making clear that the “smart” generals and admirals are those who support him, and that “dumb” or “failed” officers are those who disagree with him. And he has no compunction about leveling blistering insults—in public—against some of America’s most highly respected military leaders.

Think of how far we have come—or how far we have fallen—from the days of the Cuban missile crisis, when John F. Kennedy called his predecessor, Dwight Eisenhower, for advice. Each man knew the importance of his rank and position; more to the point, they respected each other’s rank and position. Eisenhower, who was old enough to be JFK’s father, even called him “Mr. President” and “sir” in private. Kennedy, speaking to the man who had only recently sat behind his own desk, called Eisenhower “General.”

Yes, presidents have blown their stacks when hearing things they don’t like from their military advisers. (Lyndon Johnson supposedly unloaded on his service chiefs with such fury in a private meeting in 1965 that the chairman of the Joint Chiefs thought of resigning.) The Oval Office and the Pentagon are places where the survival not only of the country but also of the planet is decided, and tempers can run white-hot.

Read: The military has become Trump’s favorite prop

And, yes, retired military officers have not helped matters. Many of them have spoken out against Trump, in ways that would have been unthinkable even a decade ago. (It is important to remember that as recently as 1992, there was considerable tut-tutting about retired Admiral William Crowe’s endorsement of then-candidate Bill Clinton.) Retired generals including James Clapper, Mark Hertling, and Michael Hayden are regular critics of the president; even retired General Martin Dempsey’s Twitter feed, which never mentions Trump specifically, seems to be a continuing sub-tweet of the president, hashtagged under “#Leadership.”

But it is the president, not the generals and admirals, who have made this unthinkable situation part of the new normal in the Age of Trump. No modern president has been so reckless in his criticism of both active and retired military professionals. When Trump said he knew more than the generals—a laughable claim from almost any civilian when it comes to military affairs—he apparently meant it. And that means he has no respect for military advice, from any direction. This, more than any personal clash, was the clear message in Mattis’s resignation.

If Trump continues on this path—and he will—we could face the most politicized and divided military since Vietnam, or even since the Civil War. Generals and admirals could be faced with betraying their professional code either by giving the advice they know will keep them in the good graces of the president or by ignoring the president’s orders and protecting their troops in the field as they think best. The rank and file, meanwhile, will become accustomed to showing up at political rallies where their commander in chief will pander to them and air his grievances against other elected officials, all while they wave banners in uniform and cheer for a growing cult of personality.

I remain optimistic. The oath of the professional officer, like the oath of the federal servant, is to the Constitution. The men and women of the armed forces have withstood greater temptations than the empty praise and illusory bribes of a desperate president. But in civil-military affairs, as in so many other areas of our national security, we shall have much damage to repair before this business is over.