During the past year, Defense Secretary Jim Mattis has sometimes seemed to be running the Pentagon with clenched teeth. He kept quiet when President Trump made decisions that Mattis thought were wrong; he sat steely-eyed in White House meetings, refusing to indulge in the idolatry toward Trump of other Cabinet members. He argued for the policies he thought were right and kept his mouth shut when he lost.

But on Thursday night, something snapped, and the unflappable Mattis did something that’s rare and precious in Washington: He resigned on principle. He didn’t make the ritual obeisance of thanking the president for giving him the job, but said instead: “I very much appreciate this opportunity to serve the nation and our men and women in uniform.”

Mattis’ admirers, and they form a small army among current and former national security officials, were unanimous Friday in praising his decision. Some even saw it as a potential turning point for a country that’s wobbling dangerously under a president who seems these days like a top that has lost its spin.

“Mattis was right to serve and to hold his tongue publicly, to be effective. But now he’s right to resign,” said Stephen J. Hadley, a national security adviser to former president George W. Bush. “As he said, the president deserves a secretary of defense who shares the president’s views. That is not Jim Mattis. Mattis is a man of principle, and he couldn’t fake it, to his credit.”

It had become painful in recent months watching Trump-encouraged speculation that Mattis had lost favor and influence. This ritual humiliation of subordinates is one of Trump’s least attractive qualities, and it was especially ugly when directed at the stoical, stalwart former Marine general. As is his practice, Mattis didn’t respond publicly, until Thursday’s zinger of the resignation letter.

“This president has a way of walking everyone over the cliff and asking them to sacrifice everything they believe in to show loyalty. Mattis just couldn’t go any further,” said a retired four-star who knows Mattis well.

Another senior former Pentagon official explained Mattis’ resignation this way: “Generals don’t do crazy.” The months of volatile Trump decision-making finally proved too much.

Mattis disagreed with Trump about so many issues, it’s astonishing that he lasted this long. He thought it was a mistake to quit the Iran nuclear agreement; he disagreed about creating a space force as a new military branch; he feared Trump didn’t understand the dangers of nuclear confrontation with North Korea and other adversaries; he disagreed with haranguing allies and trading partners; he disliked sending regular military troops to police the border.

But on all these disruptive Trump policies, he was a loyal soldier. Where possible, he slow-rolled issues. A tweet wasn’t the same thing as a formal presidential order, he advised colleagues. His standard response, when people asked how he was able to serve a man whose public qualities were so unlike his own, was that the country had elected Trump, and it was his job to serve the president.

This rationale finally unraveled this week. Mattis was appalled by Trump’s decision to withdraw troops from Syria and Afghanistan before their mission was completed. Mattis evidently viewed it as a betrayal of the warrior’s ethos of never running from a fight.

It was especially galling that Trump didn’t consult key allies before making these decisions. As Mattis said pointedly in his resignation letter: “One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships.”

A final “stake in the heart” for Mattis was Trump’s rejection of his strong recommendation of Gen. David Goldfein, the Air Force chief, as the next chairman of the Joint Chiefs of Staff. Trump instead chose Gen. Mark Milley, the burly, tough-talking army chief. To Mattis, that was a thumb in the eye.

The sublime thing about the U.S. military is that it rolls onward, through tweetstorms and turmoil. Explains Robert Gates, defense secretary for Bush and his successor, Barack Obama: “A lot of [the Defense Department] runs on autopilot, and the day-to-day stuff will continue. But anything that is controversial, that divides the chiefs, will come to a screeching halt. Because only the secretary can make people do what they don’t want.”

Gates cites the intangible quality that will be Mattis’ greatest legacy: “The morale factor — the sense that there’s someone strong leading the department who has the troops’ best interests at heart.”

— David Ignatius is a columnist for Washington Post Writers Group.