Rome’s was a surprising example of successful ethnic assimilation, Randall Collins suggests:
After about 60 BC, most of the famous authors and politicians had been born outside of Rome: Cicero, Catullus, Virgil, Horace, Livy, and Ovid all came from remote parts of Italy. During the following centuries of the Roman Empire, virtually none of the famous names were born at Rome, and they came not only from Italy but from the provinces. At least in the wealthy and educated classes — the only people that we hear about in the histories — ethnic distinctions had disappeared. Latin became the universal language throughout the western provinces; all traces of local cultural identities disappeared. In the eastern part of the Empire, where the provinces had been under Greek-speaking rulers, Greek continued to be spoken but Latin was used in official matters. With the end of a few areas of die-hard resistance, one hears no more of ethnic nationalist movements. The upper classes and the upwardly mobile, at any rate, lived their lives as Romans.
And They All Lived Happily Ever After?
Well, not exactly. The rich kept on getting richer, the poor more displaced from anything except seeking handouts. Generals became politicians and vice versa. Although the ethnic citizenship issue was settled, the struggles turned into civil wars over personal power, until domestic peace was finally established by a hereditary monarchy.
“domestic peace was finally established by a hereditary monarchy”
This is not what happened. If you go through the whole list of Roman emperors from Augustus to Theodosius (the point at which east and west definitively separated), you find exactly one and only one case where it went father-son-grandson, that being the family of Constantine.
There were several father-to-son inheritances but the sons were uniformly disastrous, and it was more a matter of “I have the power so we’ll keep it” than any formal monarchical theory. The Persians by comparison were a model of stability.
As for the overall point, it’s valid. The Roman state was perhaps the one case where a proposition nation (everybody is Roman) actually became inculcated enough in the various component populations, became accepted enough, that it actually worked. The people we call Byzantines called themselves Romans and still considered themselves Romans.
It was a process that took centuries though, and the later-stage “Roman” populations were not in any sense the same people as the Romans who founded the empire, no more so than Mexicans and Guatemalans are the same type of Americans as George Washington or Thomas Jefferson, no matter how well they play baseball.
Some other pieces you should look at on Rome, immigration, and race:
What Race were the Greeks & Romans?
Ancient Romans, Race and Immigration
Ancient Romans, Beauty and HBD
Biology of Slavery
Rome & Genetic Pacification
The Ancient Concept of Race
More stuff at humanbiologicaldiversity.com.