Well, I think the very reason that Cachimbo and SD point out for the issue of national/cultural identity being...well, if not more volatile or important, than certainly entrenched and historically-founded in Europe than in America works the other way as well.
(Not to disagree with their points, though, as they are pretty valid.)
European nations have, for the most part, had those hundreds and in some cases thousands of years to build an identity.
America's identity, it can be argued, is still being shaped to a degree, 230+ years in.
But for that very reason, what identity there IS already may be clung to even more ferociously or fanatically in America in some cases, for the simple reason that, feeling there's LESS of a string that binds America together and makes it America, those who defend such values hate to see any go, as it's in their eyes a case of losing what little they already had identity-wise.
The major identity "threads" in America, however, are on an inevitable line of conflict:
The perception of American being a predominantly-European-and-Christian nation (ie, "One Nation Under God," it was only added to the Pledge in the 1950s, but for most Americans, it resonates like it was there Day One) and the ideals of immigration and revolution.
America WAS, of course, predominantly White and Christian and European at the start...the Founders, certainly, weren't that way--plenty of deists, agnostics, and atheists in the bunch, from Franklin and Paine to Jefferson and even Washington--but the average American in 1781 had two common threads:
They were fighting a revolution together--for the most part--and they were white, of European descent, and Christian.
Flash foward to 2011, and there are reports now that within 20-50 years, whites may no longer be a 51%+ majority in the US...still likely the largest group will be whites, but still, with the huge influx of Latin Americans and Asian Americans especially, the Whte-First US Culture is vanishing quickly and in all likelihood irreversibly.
(Incidentally, that's one thing I DO like about living in California, you REALLY see that influx and DO get to see all the diversity, CA's probably one of the most diverse states in the Union...probably the biggest difference is Northern California was and remains a large site for Asian immigration, and so there are more Asian-Americans up there than down here in So Cal, where our proximity to Mexico and Latin America makes the Latin-American population larger than in Northern CA, and probably anywhere else in the country.)
The other side of that first thread, the "Christian" side, is unravelling fast as well...
Case in point--take this very site.
How many on here are atheists or agnostics, like I am?
Quite a few.
How many are religious?
Again, probably quite a few--we actually have a good mix here, I wonder what our exact numbers are, faiths vs. anti-faiths--but not at all the 99% or whatever it must have been in 1781 or even 1951 for most of America.
The bottom line to all of this:
America's "founding features" of white, European descent, and Christian are all diminishing markedly, and with THAT so many Aemricans say go "American family values."
...
"American Family Values."
Pardon me while I recover from being sick at the very sound of that phrase...
But it's true, and THAT'S whee so much of the fundamentalist and hate groups in the US come from--a fear of losing what little identity they feel America already HAS.
Those in England or another part of Europe, a question:
Suppose we granted this idea that multi-culturalism IS a threat--at least in the eyes of some--to your national and cultural identity.
You'd STIL have hundreds or thousands of years of myths and legends and kings and castles and traditions still THERE in the woodwork, you might lose an ethnic majority, perhaps, but still, there's still be the same elements of culture there all over the country in the flags and structures and art and music and all manner else that creates a cultural idenity.
America HAS no King Arthur or Roland legends, no Beowulf, no castles or old songs or centuries of traditions that make up a culture...
It's "white and Christian" to many Americans that make America America...
And so, with "less" to start with, they strike back all the more to protect what little they fear they have in the first place.