One of the ideas which has, perhaps more than any other, led to war and suffering in the modern age, is the idea that countries should have clear ethnic/national identities which define their borders. This is something that we in the the US, which has been heavily defined by immigration and thus lacks a distinct ethnic national identity, but it is something which comes into stark relief when we look at conflicts in other parts of the world.
Of these, the one that gets the most press is, of course, the conflict over the Holy Land, where different factions insist that the same ground should belong to either a Jewish State or a Palestinian State. This leads to strife because obviously if the state in a given area is specifically intended to belong to one ethnic or cultural group, then members of other groups must either leave or see themselves as living in someone else’s country.
This would work very well if various ethnic groups had spontaneously generated from the soil of different regions, but this is not the case. (After all, if you trace it back far enough, we’re all Africans.) Recorded history is one long story of migrations, conquests and assimilations.