It looks like Secretary of Defense Jim Mattis may be next in line to get the boot for disloyalty to President Donald Trump.

For some time now, Mattis has made statements and pursued policies at odds with Trump’s predilections, especially when it comes to strengthening U.S. commitments to NATO and assuring allies in Asia. But his most recent challenges have been particularly upfront, and this time Trump hit back.

The latest flap began Tuesday, when Mattis was asked at a news conference about the future of U.S. joint military exercises with South Korea. At the June summit in Singapore with North Korean leader Kim Jong-un, Trump had promised to suspend those exercises, saying they were provocative and expensive.

On Tuesday, Mattis—who hadn’t been consulted on this move at the time—made his opposition to Trump’s move clear. The suspension, he said, was taken “as a good-faith measure,” he said. Then he added, “We have no plans at this time to suspend any more exercises.”

In response, the next day, without mentioning Mattis by name, Trump issued a remarkable rebuke—an unusual four-part tweet, headed “STATEMENT FROM THE WHITE HOUSE,” which read, in part:

President Donald J. Trump feels strongly that North Korea is under tremendous pressure from China because of our major trade disputes with the Chinese Government. … Nonetheless, the President believes that his relationship with Kim Jong Un is a very good and warm one, and there is no reason at this time to be spending large amounts of money on joint U.S.-South Korea war games. Besides, the President can instantly start the joint exercises again with South Korea, and Japan, if he so chooses. If he does, they will be far bigger than ever before. As for the U.S.-China trade disputes, and other differences, they will be resolved in time by President Trump and China’s great President Xi Jinping. Their relationship and bond remain very strong.

In other words: Butt out, Mattis; Trump is the only one who decides whether to restart the exercises, and meanwhile he’s handling everything just fine.

In short, Mattis has lost his political immunity.

The New York Times’ Mark Landler quoted Pentagon officials as saying that Mattis’ comments—and press reports citing them as contradictory to Trump’s—angered officials in the White House. It wouldn’t be farfetched to imagine that Trump may also have been annoyed by Mattis’ tribute, a few days earlier, to John McCain, lauding the late senator for living a life embodying the Naval Academy’s motto, “not for self, but for country”—which could be read as a dig at Trump, whose life has embodied the opposite.

Mattis has always seemed to be out of place in Trump’s entourage. Trump chose him to be defense secretary, in large part, because of his supposed nickname, “Mad Dog.” Once in office, he was clearly surprised that Mattis—a retired Marine four-star general and combat commander—opposed torture and resisted efforts to start new wars.

Still, Mattis was extremely popular on Capitol Hill and a source of assurance among U.S. allies. If Trump ever felt inclined to fire Mattis, he might have been deterred, for much of his time as president, by the storm of panic and protest that such a move could stir up.

Trump might not feel that way any longer. The Republicans in Congress have proved themselves unwilling or unable to speak out against any outrage that Trump might commit. The allies still admire Mattis, but they’ve learned that he doesn’t reflect the administration’s views—and, besides, Trump no longer cares much what the allies think.

In short, Mattis has lost his political immunity.

Some who know Mattis say that he would not leave at his own initiative, at least not before the midterm elections. His military ethos demands at least that degree of loyalty and an aversion to politicize his actions, regardless of what he may think of Trump personally. He has also managed, under the radar, to push through certain policies—for instance, new security initiatives in NATO—that rub against Trump’s inclinations. But those avenues may be closing—hence the White House pushback to his statement on military exercises with South Korea.

The clash goes beyond the question of merely resuming the exercises. It reflects a deeper clash over how to deal with North Korea’s nuclear-weapons program and the security of East Asia. Mattis doesn’t want war—in fact, before the Trump-Kim lovefest began, he resisted White House pressure to draw up new attack plans—but he does favor continued pressure on Kim to take steps toward disarmament. On this, he is in accord with Secretary of State Mike Pompeo and National Security Adviser John Bolton. Trump is alone in believing that Kim is an honest friend with honorable intentions.

But Pompeo is more interested in kowtowing to Trump than in speaking up for U.S. interests or giving him diplomatic advice. Bolton, who has long expressed a desire to bomb North Korea (and Iran) into submission, seems to be sitting back, waiting for the moribund talks with Pyongyang to collapse.

If Mattis leaves, whether on his own or on Trump’s orders, the Cabinet will consist entirely of political lackeys or manipulators. Sen. Lindsey Graham is rumored to yearn for the job—which may explain his recent turnaround on Trump’s suitability for public office (a shift that bodes ill for what sort of secretary of defense he would be). Another aspirant is Sen. Tom Cotton, whose fervid opposition to the Iran nuclear deal would earn him points with Trump as a candidate.

Mattis hasn’t been the ideal defense secretary, but at least he’s been doing the job. He’s the last “grown-up in the room.” If Trump’s halls of power seem like a playpen run by the rowdiest kids, wait till the chaos spreads across the river to the Pentagon and across the oceans to our dealings with friends and foes.