> If you look at the borders of Europe and how they flop around over time, it becomes clear that the 'delineating' factor is mostly ethnicity, at least crudely.
No. I don't know why you would think that. Brittany was Gallo-Breton, and the two ethnicity almost cut the duchy in two. Two very, very different languages and ethnicity. I would know, i was born on the lingual border. Occitan was closer to Ligurian than to Paris' French, Normand was closer to London's, I would like to think that Liege was always speaking alsacian rather than French, and that Burgundy was speaking more French than the Armagnac Duchy was. The Basque ethnicity was always separated between two crowns (at least)
You know what defined the national borders during the nationnalism epidemic of the 19th century? Natural borders. And when none exist, you create artificial ones, not a all following ethnicity. Ukraine? Lithuania? Poland even? No natural borders => the borders cut with forests or swampsm not "ethnical divide" (or else eastern Europe would NOT look like this).
> Napoleon failed outside his borders whereas within France, it might have just worked for him.
Yes, and what was France looking like when he took power? How yeah, Savoy, Piedmont, Liguria are really French. As are the netherlands... It worked for him until he started to meddle into other kingdom's/Empire internal affairs.
> Greece is still Greece today after 2500 years, the borders are similar.
Yeah, no. The old Greece western borders stopped before that (linguistically at least), the northern border reached higher, as were the eastern borders. I know that Iran fancy themselve as persia successor, but let's be honest, their border follow more natural borders than the old Persian Empire.
If ethnocentrism were "as old as time", what about the Mughals? What about the Romans? What about the "King of Franc" ruling over a lot of non-Franc (a majority of non-Franc even)? And the Holy Roman empire? What about the Commonwealth of Poland? The truth is, ethnicity is a lot like language, and follow the same rules language used to do.