For many, and even for self-proclaimed progressives, Donald Trump’s pick to be secretary of defense, retired Marine Corps Gen. James Mattis, is a light in the darkness—a kind of oasis in the midst of a vast reactionary desert. And so it seems, all of us can now breathe a sigh of relief: After all these weeks, there’s finally an adult in the room. What’s not to like? Mattis, as has been reported, is not just a warrior, he’s an intellectual. Entire websites are dedicated to his pithy bon mots (“be polite, be professional, but have a plan to kill everybody you meet”), which, along with his...