All cultures mimic, adopt and modify aspects of other cultures. See: New York pizza, Australian beer etc. It doesn't follow that the resulting outcome is any less a valid expression of national cultural identity.
Likewise, I don't follow the connection between having a national identity and having national borders.
I once invited my neighbour over for dinner. He introduced me to his BBQ recipe which I now make all the time for my family. But he also got drunk and pissed himself on my couch. I won't be inviting him to move in with us any time soon, despite us having some culinary cultural overlap.
Well there is a stark difference between: "This is my grandma recipe, a family heritage" and "This is my drunk piss-himself-on-the-couch neighbor recipe" the first is something that can inspire you to impale yourself on a bayonet, the second is not.
In Europe much of the teaching of history in school has something to do with inventing a common national narrative, that's why everyone minimizes their own war crimes (with some exceptions after Nazism and Fascism, where Germany and Italy wanted a clean cut), and that's why France has forbidden Paths of Glory until 1975 and celebrates WWII victory.
As I was saying in another post almost every country in the Balkans has some kind of national myth about when they were an empire spanning the whole region (so does Italy about the Roman Empire). Ukrainians are very proud about Kievan Rus' (as a tool to prove superiority or precedence over Russians) and I could go on about this.
Maybe New World countries are based less on a nineteenth century national myth, with everyone being able to trace their ancestry to some immigration a few generations back, but here in the Old World this kind of half lies are still very much present.
Re: the connection between national identity and national borders is basically a softer version of the racial substitution conspiracy theory: if these immigrants swamp our country they are going to prevent us from using real pork in carbonara and add pineapple to our pizzas.
All cultures mimic, adopt and modify aspects of other cultures. See: New York pizza, Australian beer etc. It doesn't follow that the resulting outcome is any less a valid expression of national cultural identity.
Likewise, I don't follow the connection between having a national identity and having national borders.
I once invited my neighbour over for dinner. He introduced me to his BBQ recipe which I now make all the time for my family. But he also got drunk and pissed himself on my couch. I won't be inviting him to move in with us any time soon, despite us having some culinary cultural overlap.