In other words, my argument is that in order to maintain internal peace, a society needs to be mostly homogeneous. Multi-cultural societies throughout history have always had major racism and other form of group-oppression. Traditionally, in order to maintain internal peace societies have exported that conflict in order to rally the citizens around the flag (example of this would be the US).
I might be wrong about this but I have never heard, read about or seen a single multi-cultural society that have managed to remain in peace for too long. The only exception to that are societies that are ruled by dictators with an iron fist (ex. Syria prior to the war). Groups did co-exist peacefully but they had to be forced to do so.