Holland and Rome

It's a long article in the Jerusalem Post about the problems Holland is facing with one million Muslims (out of a total population of only 16 million), but worth reading. Holland amply demonstrates that tolerance only works if both sides to the equation are tolerant. In a battle between tolerance and fanaticism, tolerance loses. The whole article also put me in mind of the fall of the Roman Empire. That's the last example I can think of in which a culture actually invited in the aliens who brought about its downfall. Maybe a few other examples are Native Americans in the 16th through 18th centuries who innocently welcomed the explorers -- although they would have been forced to allow the well-armed explorers in had they not been naive enough to invite them. No, Holland (indeed, all of Europe), like Rome, is a long-standing, well-defined sovereign community that got so comfortable, it was unwilling to do its own labor. Holland (that is, Europe), like ancient Rome, invited in massive laborers from entirely alien cultures -- and Holland (that is, Europe), like ancient Rome, is now being devoured by the very immigrants it welcomed. Which leads me to a question that I'm not ready to answer -- and I would love input from others: Why did America, which invited huge numbers of people in over its life time, maintain up until recently a coherent American culture that was augmented, but not destroyed, by the immigrants? (I qualify this question by saying "until recently," because I think current intellectual trends, such as multiculturalism, political correctness, extreme feminism, etc, are finally undermining a dominant American culture in a way that centuries of immigrants never did.)