Borders may be imaginary lines on a map, as leftists are so fond of pointing out; but the cultural differences between the peoples they separate are very real.
I don't know, the cultural difference between different areas of the US can sometimes be greater than, say, the difference between the US and Canada.
Yes that's the thing: there was nothing more destructive in terms of cultural diversity than the invention of nation-states. Suddenly everybody, including people who were quite happy to identify as inhabitants of some village or small province was required to adopt the habits, culture and symbolisms of a distant capital. That is the poison of nationalism: in its very arbitrary splits between people, it bulldozes diversity of thought, and forces ordinary people to align with forces by nature opposed to their interests (I.e. The national elite).