Some have noticed recently that the ancients realized that diversity was a means to an end, namely of the power of tyrants. Guillaume Durocher quotes Aristotle on the topic of multiculturalism:
Aristotle’s ideal of citizenship, entailing civic duties and group solidarity, necessarily requires a strong common identity and a sharp differentiation between citizens and foreigners. Conversely, foreign mercenaries had no solidarity with the people, and were thus frequently used by tyrants to enforce their unjust rule:
The guard of a legitimate king is composed of citizens: that of a tyrant is composed of foreigners.
It is a habit of tyrants never to like anyone who has a spirit of dignity and independence. The tyrant claims a monopoly of such qualities for himself; he feels that anybody who asserts a rival dignity, or acts with independence, is threatening his own superiority and the despotic power of his tyranny; he hates him accordingly as a subverter of his own authority. It is also a habit of tyrants to prefer the company of aliens to that of citizens at table and in society; citizens, they feel, are enemies, but aliens will offer no opposition.
This passage brings to mind the Bolshevik tyranny in the early decades of the Soviet Union, when the government, and especially the secret police, was dominated by people from non-Russian ethnic groups.
You can also find Aristotle referring to the failure of diversity as a cause of civilization disintegration:
Another cause of revolution is difference of races which do not at once acquire a common spirit; for a state is not the growth of a day, any more than it grows out of a multitude brought together by accident. Hence the reception of strangers in colonies, either at the time of their foundation or afterwards, has generally produced revolution; for example, the Achaeans who joined the Troezenians in the foundation of Sybaris, becoming later the more numerous, expelled them; hence the curse fell upon Sybaris. At Thurii the Sybarites quarrelled with their fellow-colonists; thinking that the land belonged to them, they wanted too much of it and were driven out. At Byzantium the new colonists were detected in a conspiracy, and were expelled by force of arms; the people of Antissa, who had received the Chian exiles, fought with them, and drove them out; and the Zancleans, after having received the Samians, were driven by them out of their own city. The citizens of Apollonia on the Euxine, after the introduction of a fresh body of colonists, had a revolution; the Syracusans, after the expulsion of their tyrants, having admitted strangers and mercenaries to the rights of citizenship, quarrelled and came to blows; the people of Amphipolis, having received Chalcidian colonists, were nearly all expelled by them.
Interestingly enough, Plato observes the exact same thing, namely that tyrants import foreigners as replacements for non-compliant citizens:
And who are the devoted band, and where will he procure them?
They will flock to him, he said, of their own accord, if lie pays them.
By the dog! I said, here are more drones, of every sort and from every land.
Yes, he said, there are.
But will he not desire to get them on the spot?
How do you mean?
He will rob the citizens of their slaves; he will then set them free and enrol them in his bodyguard.
To be sure, he said; and he will be able to trust them best of all.
What a blessed creature, I said, must this tyrant be; he has put to death the others and has these for his trusted friends.
Yes, he said; they are quite of his sort.
Yes, I said, and these are the new citizens whom he has called into existence, who admire him and are his companions, while the good hate and avoid him.
It would be foolish to imagine that anything about human behavior has changed for the last 2400 years. The same tactics still work: if you want to rule forever, subjugate people by destroying their culture and importing scabs to supplant them. The EU and US have pursued the same policy since 1965.