In principle, who can argue with Mattis when, in his letter, he talks about his dual beliefs that we should have strong relations and cooperation with our allies, and should also be steadfast in countering adversarial actions by Russia and China? I’m sure POTUS would agree with this, in principle, as well.
But the question is, how do you achieve such things? POTUS’s approach to achieving these things is more flexible than the tradition approach followed by people like Mattis. POTUS is not satisfied with the status quo and is seeking a better solution, one in which allies behave like allies, and where adversaries are confronted when their actions harm our national interest, but not otherwise.
Adam Schiff could have written that “resignation” letter. It’s a gold mine for Democrats and they will run with it. As Mattis intended I guess. He’s making trouble for POTUS here with the new Dem House. No doubt.