"But it will come, and when it does, we are f***ed"

The question now facing both U.S. and foreign leaders is plain: Who will constrain Trump? Mattis was one of the few remaining officials who seemed able to twist the president’s arm out of his most rash and unwise decisions and impulses.

“Mattis is the last brake on a president that makes major life-and-death decisions by whim without reading, deliberation, or any thought as to consequences and risks,” said a senior U.S. national-security official on Thursday, who spoke on the condition of anonymity in order to talk freely. “The saving grace is that this president has not been tested by a major national-security crisis. But it will come, and when it does, we are fucked.”

While the administration has seen heavy turnover, there have been few resignations on principle, despite Trump’s frequent departures from norms. It’s also exceedingly rare for senior officials in any administration to resign in protest. There had been frequent speculation that Mattis might leave the Pentagon after some of Trump’s previous actions. The economic adviser Gary Cohn resigned over his disagreements with Trump about tariffs.