He shows that distinctive patterns of cruelty and callousness pop up repeatedly in history, cutting across times, places and political systems. He insists that ''we need to look hard and clearly at some monsters inside us,'' not to make us pessimists but as ''part of the project of caging and taming them.'' For Glover argues that human nature encompasses not just destructive impulses but ''moral resources'': humane impulses that sometimes recoil from the intentions of the monsters. The course of history, and our hopes for the future, are shaped by struggles among these impulses inside countless minds.[...]

Glover sees two countervailing moral resources. Human responses — sympathy, empathy and respect — occasionally break through in people committing vicious acts. Sometimes they are triggered by the intellect. A British World War II navigator, safely home after a bombing raid, says to the pilot, ''What about those poor sods under those fires?'' Entrenched soldiers say, ''We don't want to kill you, and you don't want to kill us, so why shoot?'' At other times they are triggered by tangible signs of a target's humanity. A soldier sees a fleeing man holding up his trousers. The mundane detail turns him from ''fascist'' to ''person,'' and the soldier loses the will to fire. An Afrikaner policeman chases a South African demonstrator, club in hand. She loses her shoe, and chivalry makes him hand it back. Their eyes meet, and he finds it impossible to club her.

The other resource is moral identity, or self-respect — the answer to the question ''Am I the kind of person who could do this?'' People sometimes resist the pressure to harm others when it conflicts with how they want to see themselves. A moral identity can come from a religion, a culture, professional mores (like the Hippocratic oath), a cosmopolitan humanism or sometimes just an insistent voice inside us.

In Glover's analysis, the horrors of the century took place when the moral resources were deliberately or accidentally disabled

I found his articulation of that disabling to be very enlightening. We need to learn those lessons of the twentieth century. Unfortunately, it seems we need to learn them over and over and over again. I share your pain and wonder as this tragedy continues to unfold.