Resignations in principle have become rare events in American government. The last one took place in 1980, when Secretary of State Cyrus Vance left the Carter Administration after getting iced out of the decision to attempt a rescue of the hostages in Iran. And even Vance didn’t leave while making as sharp a critique of the administration as the one Mattis has leveled.
Mattis’s disagreement with the president over Syria may have precipitated his resignation, but Mattis says that the difference runs much deeper. Mattis writes that he believes in respecting allies, being clear-eyed about enemies and competitors, and fostering an international order that is conducive to American security, prosperity, and values. He then — in his very next sentence — suggests that he is at odds with Trump on each of these questions and more.
We are therefore to infer that in Mattis’s view, Trump does not respect our allies, is not clear-eyed about our enemies and competitors, and is undermining our security, prosperity, and values. It is the kind of critique one could easily imagine a presidential candidate running against Trump making in 2020. I can’t really think of an American historical case that’s at all analogous. The closest one that comes to mind was the resignation of a Secretary of State 158 years ago, also in December.