Based on the excerpt, even “sideswipe” may be too strong a verb for the criticism of the president Mr. Mattis doles out. His disapproval is so veiled that it is practically shrouded. Mr. Mattis’s essay touches briefly on his interactions with Mr. Trump when the president first asked him to take the post and explains his decision to return from retirement as one motivated by a sense of duty. As to why he resigned, he offers: “I did as well as I could for as long as I could. When my concrete solutions and strategic advice, especially keeping faith with our allies, no longer resonated, it was time to resign … ”

Presumably this is a reference to his disagreements with Mr. Trump over the president’s denigration of our allies and coziness with Russia, which Mr. Mattis pointed to in his resignation letter. But in his first big appearance after his departure from the Pentagon, he still doesn’t say that outright. The closest Mr. Mattis comes to explicitly criticizing the president is declaring that “A polemicist’s role is not sufficient for a leader. A leader must display strategic acumen that incorporates respect for those nations that have stood with us when trouble loomed.” But Mr. Mattis then immediately weakens that point with a vague gesture at how “tribalism must not be allowed to destroy” the American experiment. The hostility and “mutual disdain” of both “tribes” is the problem, he suggests — not the “disdain” of any one person in particular.

All this leads to the question: Whom, exactly, is Mr. Mattis’s essay for?