Before the Nazis murdered six million Jews, they called them rats and vermin. Before the 1994 genocide in Rwanda, Hutu leaders declared that all Tutsi men, women and children were inyenzi, cockroaches. Today, similarly dehumanizing language is surging in countries like Burma, Greece, Nigeria and Iran—and history teaches that it can inspire mass violence if left unchecked.

Such inflammatory speech is launched from a variety of platforms: newspapers, broadcasts, pulpits, the Web, and even text messages. It's the content that's...