The defense secretary could no longer serve a president who no longer thinks he needs to listen to anybody. James Mattis is not an imposing man. At five-foot-nine, with a slight build, he doesn’t have the physical presence you might expect from someone whose nickname is “Mad Dog.” He doesn’t have, say, H.R. McMaster’s bull neck or booming voice. Yet Mattis loomed large over U.S. national security policy, such as it is, under this presidency—so much so that his long-expected but still-sudden resignation Thursday had Washington reporters competing to see who could dial up the most hair raising quotes warning of catastrophe ahead.

(My contribution: One former top official who speaks regularly to the White House offered only a one-word reaction over email: “Alarming.”)
When I met Mattis for the first time, at Stanford University’s Hoover Institution, the retired Marine general was still fuming over his treatment by the Barack Obama administration—he was fired as Central Command chief, basically, for urging a more aggressive Iran policy—and though our conversation wasn’t on the record, it was clear he was somebody who wasn’t to be trifled with.

Now, he’s aiming his considerable capacity for outrage at a different occupant of the Oval Office, with the stakes far higher given that the president today is, well, Donald Trump.
Every journalist in Washington knew Mattis opposed Trump on the biggest foreign policy issues of the day, be it pushing back against a revanchist Russia, managing the messy conflicts in the broader Middle East, or handling a surging China. He made it known around town that he was only running the Pentagon to protect it, if not the world, from the president, and for nearly two years he was more or less able to prevent an outright crisis.
The question was always when Mattis would reach his breaking point—when the president’s isolationist instincts, impulsive decision-making and attempts to use the military as a political weapon would push him over the edge.

It wasn’t, apparently, Trump’s deployment of U.S. troops to the Mexican border in a transparent effort to swing the November midterms that did it; Mattis went along with that. It wasn’t the president’s repeated snipes at NATO, the transatlantic alliance that has underpinned American national security for seven decades; nor was it his assiduous adoption of Kremlin talking points or his periodic eruptions at U.S. allies—it was basically all of that, Mattis made clear in his extraordinary resignation letter, which contains not a scintilla of praise for his boss and outlines several major points of disagreement.

“Because you have the right to have a Secretary of Defense whose views are better aligned with yours,” Mattis wrote, “I believe it is right for me to step down from my position.”
Obviously, Mattis couldn’t abide Trump’s sudden and apparently unilateral decision to pull U.S. troops out of Syria, where they were supporting predominantly Kurdish forces in fighting against ISIS and keeping an eye on an encroaching Iran. Word soon leaked out, too, that Trump plans to yank a big chunk of U.S. forces from Afghanistan, a flailing war effort the president has long questioned as pointless.
It’s not that these moves are indefensible—one can easily imagine a President Hillary Clinton determining that the juice wasn’t worth the squeeze and ordering U.S. forces home.

But in a normal administration, a big move like that would have taken place only after endless rounds of discussions at multiple levels of governments, arguments between agencies, and consultations with allies. There would be plans for every possible contingency, and a carefully coordinated PR rollout. Trump seems to have just ordered it done at the speed of a tweet, and it’s clear his administration hasn’t worked through the dangers that accompany any withdrawal of troops from a war zone.
On Wednesday, the administration hastily announced a conference call to brief the press on the president’s Syria decision, then struggled to explain what it was and when or how it would happen. The Pentagon pointed reporters to the White House; the White House told reporters to talk to the Pentagon. It was a level of chaos I hadn’t seen in a decade of covering U.S. foreign policy.

So it isn’t surprising that Mattis left—with his advice so conspicuously spurned, he might have realized he could no longer be effective. For years, he had ignored or slow-walked Trump’s wilder ideas, such as his reported order to assassinate Syrian President Bashar al-Assad, but this time the president doesn’t appear to have consulted him at all—and reportedly rejected his desperate final attempt to change his mind over Syria. Any Cabinet secretary would have resigned.
So what now?

Each occupant of the Oval Office claims ever-vaster powers in foreign policy, and Congress has steadily ceded its oversight powers as the complexity of conflicts, and the speed and might of the U.S. military, has increased. So those looking to Capitol Hill for a public intervention may end up disappointed. After all, Republican senators have wished away their differences with Trump on foreign policy for many months, occasionally rebuking him but generally doing little to rein him in.