Former secretary of defense Jim Mattis, a Marine’s Marine, has traded in his combat boots for tap shoes. On Tuesday, he was at the Council on Foreign Relations talking about his best-selling new book, “Call Sign Chaos: Learning to Lead” (co-written — full disclosure — with my friend Bing West), and tap-dancing around questions about President Trump. His entire book, with its praise for U.S. allies and insistence on the importance of reading (“If you haven’t read hundreds of books, you are functionally illiterate, and you will be incompetent, because your personal experiences alone aren’t broad enough to sustain you.”), reads, as Jeffrey Goldberg wrote in the Atlantic, like a giant “subtweet” of the president. Numerous questioners at the council, as at other stops on Mattis’s book tour, tried and failed to get him to turn subtext into text. As he has repeatedly done, Mattis again made clear that he doesn’t want to make the job of current officials “more difficult by a former secretary of defense speaking now, from what I would call the cheap seats.” Asked to comment on Trump, Mattis instead commented on the poisonous partisanship that pervades the entire political process. Instea...