The Weathermen. The Symbionese Liberation Army. The FALN. The Black Liberation Army. The names seem quaint now, when not forgotten altogether. But there was a stretch of time in America, roughly between 1968 and 1975, when there was on average more than one significant terrorist act in this country every week, and the FBI combated these groups and others as nodes in a single revolutionary underground, dedicated to the violent overthrow of the American...