Multi-culturism probably detroyed Rome. The empire expanded to the point where the Romans were a minority, and it was eventually destroyed by people of non-Roman stock living within the empire.
I remember reading about thirty years ago that a Japanese man of some note stated that the USA would fail in future, becaue it was multicultural, and hence not a cohesive nation. I thought at the time that his prediction would probably transpire to be correct, and I see no reason to think otherwise.
