WHEN Frazier Glenn Miller shot and killed three people in Overland Park, Kan., on Sunday, he did so as a soldier of the white power movement: a groundswell that united Klansmen, neo-Nazis and other fringe elements after the Vietnam War, crested with the bombing of the Oklahoma City federal building in 1995, and remains a diminished but potent threat today. Mr. Miller, the 73-year-old man charged in the killings, had been outspoken about his hatred of Jews, blacks, Communists and immigrants, but it would be a mistake to dismiss him as a crazed outlier. The shootings were consistent with his...