When Friends Become Enemies

Warfare is often understood as a contest between visibly distinct groups.

But history shows that the worst conflicts can occur within such groups. Pol Pot, Joseph Stalin and Mao Zedong oversaw the killing of millions of their own people. The executioners spoke the same languages, ate the same foods and practiced the same religions as their victims. Yet the bloodlust was unrivaled.

There is an uncomfortable lesson hiding in these stories. First, you have to understand something crucial: