I'm not an American citizen. I do believe that America can become an ethnicity of culture for a person over time. Even "Native Americans" (incorrectly referred to as Indians in the USA, historically) originated from Asia thousands of years ago. And before there were German, Dutch, English people, or Italian, Spanish, French people, there were Germanic peoples, Romans, Gauls, and Celtics, etc., and many of the countries that you think identify with only one, were often intertwined.
That's only if you want to get technical.