> Before "nations" people didnt regard borders, states, etc. as they do today. "citizenship" of a "nation" is a relatively recent phenomenon.
"Nationalism" had more to do with the relationship between the state and the nation, not the existence of nations. The word "nation" is very old.
Nation comes from the Latin "natio" meaning "birth, origin; breed, stock, kind, species; race of people, tribe"[0]. Thus, the essential basis for nationality is familial, a matter of common descent (as all human beings form an extended family, where you draw the line on this blurry map will depend on other factors like culture and language and ultimately the good held in common; note how Croats, Serbs, and Bosnians speak basically the same language, it is the religious and therefore cultural differences that separate them). Naturally, people migrate all the time between nations. That is normal to the degree that migration does not harm the common good of the host society. But immigration is effectively a matter of adoption. We can adopt children. We can also adopt nations.
[0] https://www.etymonline.com/word/nation#etymonline_v_2309
"Nationalism" had more to do with the relationship between the state and the nation, not the existence of nations. The word "nation" is very old.
Nation comes from the Latin "natio" meaning "birth, origin; breed, stock, kind, species; race of people, tribe"[0]. Thus, the essential basis for nationality is familial, a matter of common descent (as all human beings form an extended family, where you draw the line on this blurry map will depend on other factors like culture and language and ultimately the good held in common; note how Croats, Serbs, and Bosnians speak basically the same language, it is the religious and therefore cultural differences that separate them). Naturally, people migrate all the time between nations. That is normal to the degree that migration does not harm the common good of the host society. But immigration is effectively a matter of adoption. We can adopt children. We can also adopt nations.
[0] https://www.etymonline.com/word/nation#etymonline_v_2309