The real culprit is ingroup versus outgroup dynamics that, not unsurprisingly given human evolution, correlate with physical attributes of the groups. The solution with the best result is complete assimilation, but such thinking is taboo.
Complete assimilation is, unsurprisingly by the same group dynamics you mention, a rarity in world history given fierce resistance by all parties involved. The world today is just about overrun with cultural irredentism of all forms, and I do believe you could say that nationalism has not really come into its own until just now. To wit:

The 19th c. was the century of ideology.
The 20th c. was the century of the State.
The 21st c. will be (among other things) the century of nationalism.