According to Robert Wright in his book Non-Zero, future generations of both the conquering bordering tribe and the conquered old civilization (at least, those who survive) will derive benefits from the resulting cross-fertilization of cultures. That, he says, drives the combined civilization further up evolution's escalator. An example that comes to mind is England after the Norman Conquest.
I would dare to hypothesize that America history of immigration and cultural cross pollination could be a framework for a somewhat peaceful means of reaching the same ends.
