Alright, without turning this into a today's immigration thread, is there any society that went through a change of the dominant ethnicity without collapsing?
eg
Roman Empire
dominant ethnicity: Italics (Only Citizens of Rome until the Constutio Antoniniana -212 BC - non citizens could acquire citizenship by serving the Roman cause in various ways, but not by birth)
With the Constitutio Antoniniana, everyone within the borders gets the citizenship and Rome starts struggling with military recruitment--->hires people outside the borders, barbarization of the legions, collapse of Rome
Moving elsewhere,
Rashidun Caliphate: dominant ethnicity: arab
eventually arabs are overwhelmed in numbers by Syrian converts-->First Fitna--->Ummayad Caliphate
Ummayad Caliphate: dominant ethnicity: Syrians
At the Second Siege of Constantinople 200k ethnic Syrians die, the core of Ummayad support--->new civil war
Abbasid Caliphate: dominant ethnicity: mesopotamiam/Iraqi, subjugated by Turks, basically destroyed by Mongols