A decade ago, Kilcullen counted about 380 right-wing groups and 50 left-wing ones, many of them armed. In the early 1990s, the faceoff between the FBI and the Branch Davidians, outside Waco, Texas, left 80 people dead—and inspired Timothy McVeigh and his gang of extremists to blow up a federal building in Oklahoma City, killing 168 people. In the late 1960s and early ’70s, left-wing groups such as the Weather Underground set off bombs all over the country; police waged deadly shootouts with the Black Panthers in Oakland, California, and Chicago; and marchers for and against the Vietnam War—mainly students and hard-hat workers—clashed in violent street battles.