And Rome did sort of fall from an influx of Germanic immigrants....because the Romans were largely racist and didn't assimilate them. So in a way it is people such as Buchanan that are bringing about the fall of the United States, at least as much, if not more, than the immigrants.
The many civil wars had something to do with it too.
I note that all those who put on the Cassandra outfit when in comes to Western civilization fail to mention the paucity of instances where Western military formations acclaimed their commanding general as Caesar.