Most countries today _are_ based on ethnicities. They may not purely be of a particular ethnicity, but their basis is some ethnic grouping. There's a reason why all Germans speak German, and all Finns speak Finnish (to name a couple).

This is one of the things that's so great about America. It wasn't founded on an ethnicity; but an idea. But if you go to some of the other (more backward) countries, people still think of American as an ethnicity.