My pet theory is that the beginning of the idea of a nation start with Louise de Savoie. The intrigues she ran harshly reduced the power of dukes while giving the royal domain and it's merchants a lot of power. The narratives the French crown drew around itself and its power helped, and the consolidated territories, removing a lot of enclaves and foreign vassals (things other crowns started to do too) was to me the beginning of the 'territorial integrity' idea.
A modern falsehood. The concept of a nation is far older - it really should not come as a surprise that defining the in-group based on kinship is an ancient practice. That's what the word itself is derived from - the Latin word for birth, i.e. common birth. Only the scale has changed, and even that not as much as some like to claim: https://en.wikipedia.org/wiki/Nation
Can you really claim that nationalism existed among the majority of people when those people were illiterate, had never traveled more than 10 miles outside their place of birth, and had never met someone who they could place outside their concept of their nation?
Adrian Hastings has claimed that England's Anglo-Saxon kings mobilized mass nationalism in their struggle to repel Norse invasions. He argues that Alfred the Great, in particular, drew on biblical nationalism, using biblical language in his law code and that during his reign selected books of the Bible were translated into Old English to inspire Englishmen to fight to turn back the Norse invaders. - https://en.wikipedia.org/wiki/Nation#Medieval_nations
This was in the 9th century. More than a millennium before that, Greek city states realized they had more in common with one-another than with the Persians, and organized along those lines in the Greco-Persian wars: https://en.wikipedia.org/wiki/Greco-Persian_Wars
Even Aristotle spoke on the importance of kinship: Also difference of race is a cause of faction, until harmony of spirit is reached; for just as any chance multitude of people does not form a state, so a state is not formed in any chance period of time. Hence most of the states that have hitherto admitted joint settlers or additional settlers have split into factions; - https://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext...
> had never met someone who they could place outside their concept of their nation?
You mean like during a war? Or their cultural inheritance of history? The Jews for example have a ~3000 year tradition of passing down stories of their nation interacting with others, e.g. the story of Moses. Even someone who hadn't left their shtetl for generations would have heard tales of other nations, e.g. the Egyptians, growing up.
I'm sure you'll agree that though based on common birth, a nation is flexible - the neighbor you quarrel with becomes kin and ally when a larger threat from further away arrives. But that doesn't imply it's based on nothing, "culturally constructed" out of thin air. So your claim that people didn't travel much supports my argument, through the rootedness required for a nation to form.
