Chronicling horrific events that brought the 20th century to witness the largest number of systematic slaughters of human beings in any century across history, this work goes beyond historic details and examines contemporary psychological means leaders use to convince individuals to commit horrific actions in the name of a political or military cause. Massacres in Nanking, Rwanda, El Salvador, Vietnam and other countries are reviewed in chilling detail. But the core issue is the psychological forces behind large-scale killing, the psychology that can be used to indoctrinate normal people with a Groupthink mentality that moves them to mass murder brutally and without regret, even when the victims are innocent children. Dutton shows how individuals are convinced to commit such sadistic acts, often preceded by torture, after being indoctrinated with the belief that the target victims are unjust, inhuman, or "viral," so that they must be destroyed or will destroy society. Understanding what makes individuals commit atrocities, says Dutton, may help world powers predict and prevent such slaughters in the future.