When Donald Trump first entered the Oval Office, many military officers viewed him with favor. Some were put off by his eccentric bombast, but on the other hand, he raised the defense budget, hired one of their own (Gen. Jim Mattis) as defense secretary, and relaxed the rules of engagement—giving commanders on the ground more freedom of action—in the ongoing wars.

Now, nearly three years later, the feelings have changed. Trump’s dismissal of Mattis, impulsive withdrawal from northern Syria, persistent nose-thumbing at allies, odd kowtowing to adversaries (especially Russia and North Korea), and hold on security assistance to Ukraine for personal political motives have led these same officers to now view him with “serious misgivings,” as one retired general said with deliberate understatement.

But nothing has aroused their unease more intensely than Trump’s recent pardoning of three servicemen who had been convicted (or, in one case, had been about to be tried) for war crimes. To aggravate matters, Trump hailed one of those three, Navy SEAL Edward Gallagher, as an American hero and reportedly told advisers that he’d like “Eddie” to appear with him at campaign rallies for the 2020 election.

No one questions Trump’s legal right to issue the pardons. As commander in chief, he can pardon, promote, or demote any member of the armed services, for whatever reason he wants or for no reason at all. But many officers are worried that his undermining of military authorities and his slap at military justice will erode discipline within the ranks—and without discipline, a well-armed military can easily devolve into a murderous horde.

“You can wreck a military this way,” says a former senior Pentagon official who is now a consultant with many contacts throughout the defense establishment. “If a soldier or SEAL doesn’t like an order or thinks he’s being unfairly punished, he now has the idea that he can go over the heads of his superiors and appeal to the president, maybe by writing a letter to Fox.”

Indeed, Trump started advocating for Gallagher and the others after commentators on Fox News—and, in one appearance, Gallagher’s brother—defended them as victims of excessive political correctness. In October, Trump took up the call, tweeting, “We train our boys to be killing machines, then prosecute them when they kill!” Tuesday morning, doubling down on the pardons against a growing chorus of criticism, he tweeted, “I will always protect our great warfighters. I’ve got your backs!”

For some officers, this was the decisive betrayal. Today’s armed forces are trained to hold their fire unless necessary, and they’re expressly barred from deliberately killing unarmed civilians, as the three pardoned servicemen were charged with doing in Iraq and Afghanistan. This is especially the case in counterinsurgency wars such as these, where “winning hearts and minds” is deemed as important as killing bad guys.

It’s worth emphasizing, though, that these officers are not on the verge of revolt. They tend to play by the book (their main complaint against Trump is that he doesn’t). They will continue to follow the president’s legal orders. For the most part, they refrain from speaking ill about him on the record (and many hold their tongues even off the record).

However, a few commanders and officials are beginning to speak out. The recently ousted secretary of the navy, Richard Spencer, who said last week that he would resign if Trump reversed the military court’s rulings and who was soon after fired at Trump’s insistence, wrote in his departing letter, “I no longer share the same understanding with the Commander in Chief who appointed me, in regards to the key principle of good order and discipline. I cannot in good conscience obey an order that I believe violates the sacred oath I took … to support and defend the Constitution.”

(The circumstances of Spencer’s dismissal were complicated: He went around Secretary of Defense Mark Esper’s back to work out a deal with the White House on Gallagher’s fate, forcing Esper to fire him. But the root cause was Spencer’s disagreement with, and public challenge to, Trump.)

In any case, Spencer, a former Marine aviator, was following a trend of former officers speaking out against the president. In October, retired Adm. William McRaven, former commander of U.S. Special Operations Command, the officer who planned the raid on Osama bin Laden, wrote an eye-popping op-ed for the New York Times, assailing Trump’s attacks on America’s values and institutions, concluding that “if this president doesn’t demonstrate the leadership that America needs, both domestically and abroad, then it is time for a new person in the Oval Office … the sooner, the better. The fate of our Republic depends upon it.”

This week, retired Adm. James Stavridis, former supreme allied commander of NATO, wrote in Time magazine that Trump’s pardons display “little understanding of the military” or its “code of conduct” and that they will have “a corrosive effect” on “the credibility of the chain of command, battlefield behavior by other personnel in similar circumstances, and perceptions about the ethical conduct of the U.S. military globally.”

On Tuesday, Richard Danzig and Sean O’Keefe, former secretaries of the navy—the service’s top civilian leader—under Presidents Bill Clinton and George H.W. Bush respectively, wrote in a New York Times op-ed that Trump’s endorsement of a war criminal “will be taken as a representation of our values” and “our own troops, many of them teenagers, will be misled by the president’s sense, or lack of sense, of honor.” They concluded, “His values are not those of our military. It will do grievous damage to our armed services if they become so.”

Perhaps no other president in American history has aroused as much distaste or distrust among military officers as this one. No one is comfortable with this arousal. The officers I’ve spoken with have no desire to get involved in politics, much less in a confrontation with the president. No one can predict how this tension plays out. No one even wants to hazard a guess. Trump is stomping on untread ground. We all have cause to be nervous.