I dont get the impression that Rome was unable to assimilate the barbarians, it was more that they chose not to though the reasons are never clearly specified that I know of.  I think there is only a rough symmetry between ancient Rome and today.  That and the fact that I don't classify the US as an imperial power anyway.Have the Jews ever fully assimilated into any society?  I don't think so, the difference with them is they try to live alongside the society where they live instead of trying to change it or themselves.  They accommodate to society rather than assimilate but it is not as disruptive as outright refusal to assimilate despite what hundreds of years of anti-Semites have had to say.As to the Romans, why they chose to not assimilate the barbarian tribes is an interesting question, and one I don't have the answer to.  I wish I had time to study it more right now because it is very intriguing.  Unfortunately, I am knee deep in Thesis research and have only limited time to research anything outside of the Seven Weeks War and Battle of Koniggratz.