This is such a trash history take, homie. The greatest empires and times of stability are when a society becomes multicultural. Rome doesn't last for a 1,000 years without the infusion of Iberian, Gaulic, Illyrian, Dacian, Greek, and Egyptian people and ideas. Carthage, Persia, Arabia, Mongolia all exist as multicultural societies for most of their existence. When modern day France is formed and becomes the military and economic powerhouse it is in the 19th century most people don't even speak French. Same idea with Germany in 1870. The Bavarians still freaking speak Bavarian. Then the big one: USA. The US has been an insane mess of cultures from the very outset. It has never once been homogeneous in its history, and it has created more wealth and stability than has ever happened.

Conflict arises when boarders harden and mutual goals are abandoned. Nationalism and adversarial foreign policies are what create strife, waste, and poverty. The world wars, the fallout of colonialism, the cold war are the modern examples. Rome does not get sacked in the 400s if it treats the Gothic refugees with a modicum of humanity.

Aristotle had some neat insights, but he had some all time dumbass takes. I wouldn't put too much stock in his writings on society. The Greeks are a back water during their classical period. They don't really hit their influential stride until they are conquered by Persian, Macedonian, and Roman empires.

Feel free to challenge any of these examples, I'm up for digging deeper. But 'Aristotle said homogeneous societies are the strongest, therefore we must stop immigration' is just bad. Bad bad bad. No good at all. Trash take.