Twice in a 24-hour period, the president of the United States took to Twitter to endorse violence. The first time, at 12:00 a.m. on Thursday, he retweeted a video in which a supporter says, “The only good Democrat is a dead Democrat.” Then, early Friday morning, as protests over the police killing of George Floyd erupted in Minneapolis and turned violent, Donald Trump tweeted that “THUGS” were “dishonoring” Mr. Floyd’s memory. He concluded with a chilling warning, “when the looting starts, the shooting starts.” Trump is by no means the first American president to talk tough or to threaten the...