It isn't tribalism, otherwise Angles, Picts, Scots, Germanics, Celts would have tried to decimate others when they have managed to, on the whole, exist peacefully side by side, usually through mutuality in modern times.
You are talking of empire building and religious war for dominance. But the overwhelming majority of religious wars have always been about resources and profiteering. These things don't happen by accident, they are cultivated and designed, by the very leaders of each nation or people. If you want to look at it from this angle then you have to see it and recognise it for what it actually is, a class war.