Your question seems to imply that the U.S. is not a nation and does not have it's own culture. Just as the French are French, the English are English, Americans are Americans.
I assumed your use of the word “nation” was in the old fashioned sense. That is that a nation is a groupe of genetically similar people, not a political boundary on a map. In that sense, americans are not a nation and our culture is not a culture. It is a hodgepodge of non-culture.
“Americans are American”
It used to be that way.