Hi! We have a section on the Fall of Rome in our FAQ. We've discussed several theories there (including the notion that the West never "fell!"), and of course (especially on this topic), there is plenty more talking to be done!

More directly to the point, Demandt compiled a list of 210 proposed reasons Rome fell; multiculturalism isn't on it, at least in those terms. If "multiculturalism" is meant to mean some sort of loss of national character, that does make the list (peculiarly under the term "Decline of Nordic character"), which I hope I showed sufficiently ridiculous to be discounted here.

I'm going a little bit away from the question here, so I hope you forgive me. I see more and more (especially in /r/politics) the collapse of the Roman empire, or the fall of the Republic, being referenced for modern political purposes. This is a drastic misuse of history. America is not Rome, nor is the EU, nor is any other place. "Multiculturalism" is a term that only makes sense in a world with nationalism, and Rome, however closely it approximated a nation-state (quite closely, I argue) did not have nationalism as a concept to frame a problem of "multiculturalism." Rolling out Rome as an example of political or physical danger for a modern phenomenon is intellectually lazy, or dishonest, or both, and I wish people would stop it.

That's not to say studying Rome is not a political activity - it is terribly political (pace, my Modern Western Civ prof!). It's a very intense study of the relationship between power and individuals, power and groups, etc. And yes, how you understand those relationships should shape your politics, but to use Rome as an analogue for any of the problems we have now is a gross simplification of both the Romans' problems and our own.

The argument goes that Roman Feoderatii were given land in exchange for service, but in the end proved less than loyal. In time the Magister Militum was drawn from the Germanic warlord of the time, and the Ostrogoths and Lombards declared themselves kings in their own right. In that light, competing Nationalisms could also be argued for the Downfall.