People don't fight over their differences. They fight because they are the same, and they want the same things. Not because they need the same things (food, sex, scarce material goods), but because they want what will earn others' envy.
Humans don't know in advance what to choose, so they look to others for cues. People can desire anything, as long as other people seem to desire it, too: that is the meaning of Girard's concept of "mimetic desire." Since people tend toward the same objects of desire, jealousy and rivalry are inevitable sources of social tension and eventually wars.
Curious to find out how well his "mimetic theory" of imitative behavior might explain the human past, Girard studied anthropology and myths from around the world. He was struck by another series of similarities: myth after myth told a story of collective violence. Only one man can be king, the most enviable individual, but everyone can share in the persecution of a victim. Societies unify themselves by focusing their imitative desires on the destruction of a scapegoat which is the origin of ritual sacrifice and the foundation of archaic religions and, again, wars.
So much to listen to, so much to read, so much to think about! Rene Girard, where have you been all my life?