Already this week, a letter from 50 respected Republican foreign-policy gurus announced that they would not support Donald Trump; Sen. Susan Collins (R-Maine) wrote an op-ed declaring that she would not vote for Trump; and Trump once again roiled the race when he told a packed rally: “By the way, and if [Hillary Clinton] gets to pick her judges, nothing you can do, folks. The Second Amendment people, maybe there is, I don’t know. But I’ll tell you what, that will be a horrible day.” The Clinton team, not surprisingly, took this as an invitation to commit violence. Michael Hayden,...