All new nations go through a period of migration followed by a period of coalescence around a shared identity. We saw it in Britain as the Angles, Saxons and Jutes became the English. The trouble with America is that it has never ceased its migration period.
So American identity has constantly been in flux. The original folkish WASP America made way for a multi-ethnic European America, which made way for modern multicultural America. No wonder Americans have such identity crises.
America will be remembered as the failure of liberal or civic nationalism; the failure of the Melting Pot. It has become a mess of a nation, with different ethnic interests using the State like a puppet for their own insular interests. Jews for Israel, blacks for welfare, etc.
If America is to survive then it will need a new, both inclusive and exclusive, American cultural nationalism. However, this will be terribly difficult in such a divided nation. Balkanisation and segregation is more likely, as America becomes a South Africa-style failed state.