Jim Mattis had enough.

The four-star Marine general and secretary of defense submitted a blistering letter of resignation on Thursday after President Donald Trump unilaterally declared that he was pulling U.S. forces out of Syria and signaled he was planning a troop drawdown in Afghanistan as well.

“The most decorated Marine and thought leader on the military basically just said, ‘Fuck this,’” a source close to Mattis told The Daily Beast.

The retired general’s surprise decision, coupled with a brutal letter to Trump that made clear his displeasure with the president, prompted longtime critics of the administration—as well as some weary allies—to declare the White House’s Era of Adults to finally be over.

Senate Minority Leader Chuck Schumer (D-NY) said “there is chaos now in this administration,” and Senate Majority Leader Mitch McConnell (R-KY) said he was “distressed.” Sen. Chris Murphy (D-CT) declared it a “national-security crisis.”

One former high-ranking member of the military told The Daily Beast that Mattis’ resignation over policy differences is an indication of how thoroughly the Trump administration has politicized the Pentagon, which has historically viewed itself as apolitical out of necessity.

“I do not know how men of honor can operate in the divisive political environment that has been created by this administration,” retired Brig. Gen. Don Bolduc told The Daily Beast in an email.

“I am surprised it did not happen sooner. I am also surprised that others remain under these less than honorable conditions.”

One former senior administration official told The Washington Post that Mattis’ exit would spark “an intervention.”

“Jim Mattis just sent a shot across the bow,” the former official said. “He’s the most credible member of the administration by five grades of magnitude. He’s the steady, safe set of hands. And this letter is brutal. He quit because of the madness.”

Former senior Pentagon officials said that Mattis likely decided to get out while he could with his reputation intact.

“I think Mattis is reading the tea leaves here,” one told The Daily Beast. “He’s leaving in part because he knows what’s coming and he wants to be able to work after this craziness ends.”

The stage for the latest high-profile departure from the White House was set with Trump’s Syria announcement on Wednesday. The next afternoon, Mattis reportedly met with the president one-on-one in the Oval Office in an attempt to get him to change his mind.

When it failed, he told Trump he was resigning and then, according to The New York Times, asked aides hand out 50 copies of the letter at the Pentagon.

The letter—which was made public minutes after Trump announced Mattis was retiring in a tweet—contained not one word of praise for the commander in chief. Instead, Mattis strongly suggested that Trump’s foreign-policy strategy was dangerously wrong.

“My views on treating allies with respect and also being clear-eyed about malign actors and strategic competitors are closely held and informed by over four decades of immersion in these issues,” Mattis wrote, adding that “I believe we must be resolute and unambiguous in our approach to those countries whose strategic interests are increasingly in tension with ours,” including China and Russia.

“Because you have the right to have a Secretary of Defense whose views are better aligned with yours on these and other subjects,” Mattis continued, “I believe it is right for me to step down from my position.”

He said his resignation will be effective on Feb. 28, 2019, in order to “allow sufficient time for a successor to be nominated and confirmed.”

White House officials who spoke with The Daily Beast acknowledged that Mattis’ views likely differed dramatically from the president’s on the planned Syrian troop withdrawal, and that a decision to do the same in Afghanistan would strain that relationship even further. But they set Mattis apart from scorned former Cabinet officials such as former Secretary of State Rex Tillerson, even in spite of the former’s tersely worded resignation letter.

Asked whether Mattis should expect to endure the same ridicule that has befallen Tillerson, whom former White House Chief of Staff John Kelly fired while Tillerson was on the toilet, one official said, “Probably not. To be honest, he did what everyone in the admin should do if they disagree with DJT. He packed up and said what he wanted to say instead of anonymously bitching to the media.”

The announcement came mere hours after Sen. Lindsey Graham (R-SC) implied on CNN that Mattis was opposed to Trump’s decision to withdraw all U.S. troops from Syria.

“Secretary Mattis is firmly in the camp of the job in Syria is not yet done, that abandoning the Kurds now will hurt us down the road,“ Graham told anchor Kate Bolduan, adding that “ISIS could and probably will come back. And I think that’s the universal view of both [Secretary of State Mike] Pompeo and Mattis.”

Mattis, Pompeo, and National Security Adviser John Bolton have all reportedly advocated against the sudden Syria drawdown, an announcement that has worried prominent American neoconservatives, but delighted the Kremlin.

Retired Army general, CIA director, and Mattis’ longtime friend David Petraeus told The Daily Beast that Mattis has been “a hugely important member of the U.S. national-security team, a very significant figure to our allies around the world, and a uniquely experienced—and respected—leader for our men and women in uniform around the world,” adding that Mattis’ actions at the Pentagon “advanced our national interests in a thoughtful, determined, and effective manner.”

Retired Sgt. Maj. Carlton Kent, who served in Iraq with Mattis, said his phone has been “ringing off the hook” with calls from worried service members and veterans since the news broke.

“They were concerned that he was the right man for the right job, and now he’s transitioning,” Kent told The Washington Post. “They thought he was leading the department in the right direction. And it was combat-hardened warriors saying that, people who have witnessed him in combat.”

Bolduc said Mattis was smart to have a letter of resignation to make public as soon as Trump made the announcement on social media. “He has watched others go under humiliation, and he was prepared to pre-empt this from happening to him,” he said.

A sitting secretary of defense publicly declaring his intent to resign over political differences with the president is almost unprecedented—and Capitol Hill responded to the news in blunt terms.

“Secretary Mattis was one of the few symbols, the few items of strength and stability in this administration,” Schumer said. “Everything that indicates stability, everything that indicates strength, everything that indicates knowledge, is leaving this administration. General Kelly. General Mattis. So many others. McMaster.”

“I am particularly distressed that he is resigning due to sharp differences with the president on these and other key aspects of America’s global leadership,” McConnell said in a statement. “I urge him to select a leader who shares Secretary Mattis’ understanding of these vital principles.”

In a high-decibel appearance on CNN shortly after Mattis’ resignation was announced, White House adviser Stephen Miller called the resignation an opportunity for the nomination of a secretary of defense “who will be aligned with the president on these critical issues.”

“Are we supposed to stay in Syria, generation after generation?” Miller shouted. “Let us not spill American blood to fight the enemies of other countries.”

During his two-year tenure as the nation’s top military official, Mattis had largely cultivated a positive relationship with the president, despite disagreements with Trump that extended beyond those stated in his letter of resignation. When the president demanded a military parade in Washington, D.C. that would mirror one he saw during a state visit to France, Mattis told reporters that the proposal was an indication of the president’s “affection” for the military, which largely reviled the idea.

In April 2017, Trump called Mattis about a horrific gas attack by Syrian President Bashar al-Assad on rebels.

“Let’s fucking kill him!” Trump said, according to the Bob Woodward book Fear.

“Let’s go in. Let’s kill the fucking lot of them.”

Mattis, according to the book, told Trump he would get right on that plan—then hung up the phone, turned to a senior aide and said, “We’re not going to do any of that... We’re going to be much more measured.”

In October, Trump remarked on 60 Minutes that Mattis “may leave” his administration and that the retired Marine Corps general was “sort of a Democrat.” But Mattis claimed he had been personally assured by the president that his job was “100 percent” safe.

“We have never talked about me leaving,” Mattis said at the time.

—With reporting from Noah Shachtman, Lachlan Markay, Spencer Ackerman, Erin Banco, and Swin Suebsaeng