Same way macedonian became one out of some southern slavic people. Bunch of people from different ethnicities mixing in new geography creating new dialects brings about people identifying as something new (with a barely related name sometimes)
When asked my ethnicity, I say American. My ancestors were primarily German. Am I German? No. I was born and raised in the US. My parents were born and raised in the US. My grandparents were born and raised in the US. At least half of my great-grandparents were born and raised in the US. All four of my grandparents were, culturally, entirely American (even the ones with parents who were born in Germany) as am I.
how do you differentiate american and english? or from the immigrants of the germans and the scotts or anyone else who doesn't consider themself americans now after so many generations?
