It's funny, in the US no one is "American" even if they're heritage goes back 5+ generations of being in the US, people have to identify themselves as whatever ONE of their ancestors was.
I know many people who will say "I'm Irish" because there name is O'Hara. That is probably true, but they've married with so many people of other descents that someone from Ireland would most likely laugh at them.
However, my wife's grandmother was born in Sicily but immigrated to the US, so she really is Italian. Her kids half Italian, and my wife quarter Italian. So I have no issue with my wife saying she's of Italian descent, because mostly she is.
Me? Well my Mother's adopted so I'll never know 100%, that said I consider her adopted parents as my grandparents so in polite company I'll just say what they were (if I know).
My great-great grandfather on my Father's side had a British father and a German mother. When he moved to the US he changed his name to his mother's maiden name because he killed someone and was trying to avoid the law. This was in the late 1800's.
We don't know of what ancestry his wife was.
My father's mother has a German maiden name so we assume somewhere in her heritage there was a German. But, it could have been long ago, or mixed with any other heritage.
My mother's adopted Father? His grandparents came from Ireland (and for some reason settled in Wisconsin).
My mother's adopted Mother? She knew (Alzheimers) but never talked about her heritage much. Been in Southern Virginia long before the Civil War, so most likely British of some variety.
But since the last of my ancestors to hit the shores of America was 4 generations ago, and since we don't know a great deal about many of them, I'm American.
I'm not a "native", but I figure we've been here long enough to be comfortable with that setting.