
In 480 B.C.E., Xerxes, the King of Persia, led an invasion of mainland Greece. Its success should have been a formality. For seventy years, victory--rapid, spectacular victory--had seemed the birthright of the Persian Empire. They had swept across the Near East, shattering ancient kingdoms, storming famous cities, putting together an empire which stretched from India to the shores of the Aegean. Xerxes ruled as the most powerful man on the planet. Yet somehow, astonishingly, against the largest expeditionary force ever assembled, the Greeks managed to hold out. Had the Greeks been defeated in the epochal naval battle at Salamis, not only would the West have lost its first struggle for independence and survival, but it is unlikely that there would ever have been such an entity as the West at all. Historian Holland combines scholarly rigor with novelistic depth and finds extraordinary parallels between the ancient world and our own.--From publisher description.
Recently I read a book called Persian Fire by Tom Holland. One detail which struck me as very interesting was how the first democratic ruler of Athens, Cleisthenes, solved the tribalism problem. Here tribalism is the perfect word as the average Athentian citizen of that time associated with one of the 10 tribes based on surnames. His solution was to invent 150 demesnes and have people arbitrarily assigned to them. He also invented new surnames.
Something similar is happening in contemporary Singapore. Every neighbourhood must not have more than X% of any ethnic group.
My point is that policy can solve this. Might make many people unhappy, like Finns who are forbidden to live in some zip codes because there are too many Finns, but policy is definitely a way to approach this problem (if it is a problem, nothing wrong with mono-cultural societies who do not want foreigners).