What used to be a republic which consisted mainly of Romans at the start eventually became ethnically diverse as time went by as people from the different conquered lands of Rome (e.g. North Africa, Eastern Europe, the Middle-East, & etc.) migrated & settled in Rome itself including the barbarians after sacking Rome!

It is then said that with the differences in ethnicity lies different values & thus, the differences in people leading to divisions & eventual decline.

Now some people say that the same fate is befalling the U.S. nowadays, that the increase in ethnicity is what will lead to its eventual decline.