Every time some high-level member of the Trump administration leaves, the staff at FiveThirtyEight debate whether it’s a big deal — and therefore whether we should cover it. Sometimes the consequences of these departures are over-hyped. Sometimes the consequences aren’t clear, so there’s not much to do but speculate. But Secretary of Defense Jim Mattis’s resignation on Thursday is a big deal. A really big deal.

First and foremost, Mattis quit in protest, naming President Trump’s Russia policies along the way. The reasons behind most departures from the Trump administration up to now have been kind of opaque. A person quits or resigns, and we’re not totally sure why. They rarely give direct reasons for their departure — we’re left only with unnamed sources speculating in news stories. Mattis’s exit was different. On Department of Defense letterhead, he explained that he was stepping down not because he was tired of government service or wanted to spend more time with his family, but because he disagreed with Trump on significant policy issues. Mattis said that the U.S. needed to be more confrontational with countries who embrace an “authoritarian model,” and named Russia and China as those countries. The obvious implication is that Mattis basically agrees with Trump’s critics outside of the administration who believe the president is insufficiently critical of Russia in particular.

Thinking about U.S-Russia policy, the timing of this resignation is especially interesting. Trump — reportedly over Mattis’s objections — announced on Wednesday that all American troops would leave Syria. Trump may have other policy reasons for this shift (more on that below), but the U.S. withdrawal was something Putin wanted and subsequently embraced. It was the latest instance of Trump aligning U.S. policy with Russian interests — and was perhaps a final step too far for Mattis.

Second, the secretary of defense is a hugely powerful job, and Mattis lent the Trump administration gravitas in foreign policy. Ultimately, if the U.S. were to be attacked or to attack another nation, the defense secretary would be coordinating U.S. actions along with the president. We are not talking about who’s in charge of White House PR, or who’s responsible for taking questions from reporters at White House briefings. Secretary of defense is a big, important role. And Mattis had credibility with some of Trump’s critics who doubted the president’s judgment and ability to make key foreign policy decisions. That credibility is gone now.

More generally, Mattis is the last to leave among a group of generals who were perceived as restraining Trump. It was never clear how much of Trump’s unorthodox foreign policy views were truly restrained by former National Security Adviser H.R. McMaster, outgoing Chief of Staff John Kelly or Mattis. But it’s safe to say there was some restraining going on. Mattis and other senior officials took a much more antagonistic stance toward Russia rhetorically than the president, for example, and even pushed official government policy in that direction. Mattis, in particular, seemed to favor a more traditional U.S. foreign policy, allying with NATO and Europe and being more distant from Russia.

Third, Mattis’s resignation is the latest sign of a fissure between Trump and the Republican establishment. In filling out his government in early 2017, Trump made several appointments that showed powerful constituencies in the Republican Party — the military, congressional Republicans, Wall Street — that he respected and would work with them. He picked generals to run his foreign policy. He named the Republican National Committee chair, Reince Priebus, as his chief of staff and a former top executive of Goldman Sachs, Gary Cohen, as his top economic adviser. But Trump seems less interested these days in placating establishment interests. He is pushing towards a government shutdown, an idea GOP leaders on the Hill haven’t embraced. He is rewriting U.S trade deals, despite Wall Street’s opposition. Now, he is parting ways with Mattis, who is well-regarded by the Washington establishment.

Trump has benefited from strong support from Republicans on Capitol Hill, particularly the collective GOP decision to basically shrug off many of the scandals surrounding Trump and his current or former allies. But parting ways with someone like Mattis, who is well-respected among congressional Republicans, is not helpful in maintaining those alliances, particularly with some Republicans already questioning Trump’s political judgment after the 2018 midterms. Mattis sailed through his Senate confirmation process more easily than all but one of Trump’s Cabinet nominees:

Mattis enjoyed bipartisan support in Congress U.S. Senate vote for Trump Cabinet secretaries who were confirmed Votes Appointee Secretary of … No Yes Shulkin Veterans Affairs 0 – 100 Mattis Defense 1 – – 98 Chao Transportation 6 – – 93 Kelly Homeland Security 11 – – 88 Perdue Agriculture 11 – – 87 Wilkie Veterans Affairs 9 – – 86 Ross Commerce 27 – – 72 Zinke Interior 31 – – 68 Perry Energy 37 – – 62 Nielsen Homeland Security 37 – – 62 Acosta Labor 38 – – 60 Carson Housing & Urban Development 41 – – 58 Pompeo State 42 – – 57 Tillerson State 43 – – 56 Azar Health & Human Services 43 – – 55 Mnuchin Treasury 47 – – 53 Price Health & Human Services 47 – – 52 DeVos Education 50 – – 51 Source: u.s. senate

Trump is losing one of Congress’s favorite members of his administration.

Moreover, Trump is reportedly removing about half of the U.S. forces currently in Afghanistan in addition to pulling American troops out of Syria. In wanting fewer U.S. forces deployed abroad, Trump is angering more hawkish figures in his own party — not only Mattis, but also congressional Republicans such as Sen. Lindsey Graham of South Carolina. Trump campaigned on deploying the U.S military in fewer places around the world, an unusual stand for a Republican presidential nominee. And he may be acting on that now, after previously embracing Graham and Mattis’s more hawkish vision.

Finally, and relatedly, Mattis’s exit fits a pattern of Trump getting rid of internal rivals and promoting loyalists. Ever since Trump assumed office, there’s been this idea that certain officials serving in his administration viewed their job as resisting Trump. One of them even wrote an anonymous op-ed in the New York Times laying out this view. But those wary of Trump’s agenda may not be long for his administration. Secretary of State Rex Tillerson, United Nations Ambassador Nikki Haley, Cohn and McMaster were all among those with reported policy disagreements with Trump. They are all now former members of the administration. To replace them, Trump seems to be picking people who agree with him — or at least people who are more careful to make sure their disagreements don’t show up in Axios or The New York Times.

All these points reinforce an overarching fact of U.S. politics at the moment: Trump, despite midterm results that suggest American voters are broadly unhappy with this leadership, is not looking to rethink his approach as president. His post-midterm actions have included:

Firing his attorney general, then picking a temporary and then permanent replacement who both had voiced skepticism of special counsel Robert Mueller’s investigation;

Temporarily revoking the credentials of a reporter he dislikes;

Embracing the possibility of a government shutdown;

And now, disagreeing with his secretary of defense enough that he resigned.

Trump may win or lose re-election in 2020. But whatever happens, he is doing it his way.