What transformed moral citizens into guillotine cannibals during the French Revolution and the Great Reign of Terror? The answer, argues Eli Sagan, is the same force that has killed millions of people in the 20th-century ideological terror. This book offers a comprehensive explanation of the gruesome Terror, its causes and its consequences for the modern world. The French Revolution introduced ideological terror to the world, but Hitler and Stalin were among its 20th-century perpetrators, maintaining its survival.