Stalin did not act alone. The mass executions, the mock trials, the betrayals and purges, the jailings and secret torture that ravaged the Soviet Union during the three decades of Stalin's dictatorship, were the result of a tight network of trusted henchmen (and women), spies, psychopaths, and thugs. At the top of this pyramid of terror sat five indispensable hangmen who presided over the various incarnations of Stalin's secret police. Now, in his...