There are a lot of reasons to support the nomination of retired Marine Gen. James Mattis as secretary of defense. He's an enormously respected and talented general. He's a scholar and an impressively well-read man. He supports America's alliances and is clear-eyed about America's challenges, especially Russia and Iran. If anyone has the intelligence and backbone to curb some of Donald Trump's worst instincts, it's Mattis. But there's another big reason why I want Mattis to get the job: He might be the only man with the legitimacy — and the guts — to take on the cancer eating away at the U.S.