My gut reaction was “nothing“ because too often it seems that way. I think @Moxie answered well by citing the Constitution. While we differ in interpretations of certain sections, I’ve never known anyone (at least not personally) who has argued for its destruction.
But beyond that? Maybe nothing.
By far the most unity I’ve ever experienced was in the wake of 9/11. It was a short-lived feeling, but a kind of peacefulness amid the tragedy.
While Moxie is correct that we’re the proverbial melting pot, that very nature is so divisive. There is a level of identity-schizophrenia within the US. The person who wears an American flag T-shirt, is the same person who, when asked their nationality, will say they are English, or Irish, or Italian, or Eastern European, or whatever. They will *not* say they’re American, even though their grandparents were born here. We’re a country of non-American Americans.
That's an interesting observation about dual nationality - and I am one myself. There is a theory that you can't really have a lasting nation without shared culture, values, language and laws, i.e. a divided house will always fall. That's why I think the West is facing very troubling times. The populations of China and Russia seem from the outside far more cohesive.
I watched an interesting segment where an American celebrity was asked about being African-American and she responded that she was American and didn't want to differentiate on colour or creed. The interviewer seemed rather unhappy with her response. I can't remember her name, but found it quite interesting.