The nation escaped an unspeakable tragedy that would have been profoundly destabilizing for the country by perhaps a few centimeters. As it is, what happened in Butler, Pa., is godawful enough. A gunman tried to assassinate former president Donald Trump and grazed his ear with a bullet, while killing one rally-goer and seriously injuring others. This was a heinous act of violence and an attack on American democracy. Trump’s reaction was truly extraordinary and will long be remembered. After Secret Service officers threw themselves on the former president and then picked him up to begin to take him to safety,...