After Martin Luther King Jr. was shot dead 50 years ago as he stood on the balcony of a motel in Memphis, Tenn., riots broke out in more than 100 cities. There were also reports of violence on college campuses and even on military bases overseas, where some black soldiers refused to report for duty.

Federal troops were sent to Baltimore. In Chicago, Mayor Richard J. Daley ordered police to “shoot to kill” arsonists and “shoot to maim” looters. In Washington, so many fires were set that you couldn’t see the U.S. Capitol because...