Here is a great comparison of Rome and the USA which was posted on another forum:
" Immigrants and non-Roman citizens WERE a tremendous problem that helped pave the way for the destruction of the empire. Originally, only Romans could be citizens. And, only Romans were expected to serve in the army. At a certain point, the need for troops began to outweight the citizen contribution, so they enacted a program very similar to our own whereby you could earn citizenship through military service. As the empire continued to grow, non-citizens were increasingly gaining positions of power and influence and some had closer loyalties to their ethnic brothers than to Rome. At various points in history the emperors conferred Roman citizenship upon great numbers of people that they had conquered.

These people influenced the direction of Roman history. As citizens of Rome they were entitled to the same priveleges that ethnic Roman citizens were. They took jobs and land that would normally have been reserved for the traditional citizens. Barbarian tribes began to frequently invade and settle in Roman lands, demanding concessions and various degrees of autonomy in exchange for not pillaging even more. The effect was that the population of the Empire began to become increasingly loyal to their own ethnic groups rather than Rome. Many of the later Western emperors were truly incompetent, but many of the problems that they bungled were related to this immigrant population.

As more and more provinces rebelled and as the barbarian tribes became more and more powerful (and wanted to move further and further south) Rome became increasingly dependent on soldiers that were either foreign or new citizens. It's hard to tell a foreign born general what to do when he has twenty thousand troops under his command. Immigration did not bring down the empire. But it certainly was a major contributer to the political, economic, and military problems that eventually brought the western empire down. The dillution of "Romans" from the Roman economic system caused great strain on the empire and helped to create a situation that the emperors were not able to deal with.

I think that the Rome/USA comparison that is made is on target. Immigration and "citizenship" are issues in both examples, the US and Rome. It creates problems for both the existing citizens and the new comers. It dilutes the national identity of both Romans and Americans. It helps to create a strong underclass that, eventually, will want to be on top. There are many, many valid comparisons."