> Why do you think that after 1000 years of war (in the case of the 'Germanics' - you could say 2000 years, i.e. against the Romans) - that places like 'Germany' still exist as a relatively coherent entity? Even when they were not 'a single state' they were allied.
When has "Germany" been a "a relatively coherent entity"? The "Germanics" the Romans fought against have just as much in common with today's Germans as they do with the French or the Dutch–the French get their name from a Germanic tribe (the Franks), the principal historical difference between France and Germany is that the Germanics who invaded the Roman Empire ended up adopting the language of the people they had conquered, whereas those who remained beyond its borders kept their original Germanic language.
Moving forward, what we call "Germany" today is just the Prussian-dominated part of the Holy Roman Empire–"Austria" being the other main component–the separation of "Germany" and "Austria" is fundamentally due to dynastic politics (the Hohenzollern dynasty ruling Prussia vs. the Habsburg dynasty ruling Austria) and religion (the former being dominated by Protestants, the later dominated by Catholics)–the differences between Austrians and Germans are really no greater than the internal differences within each country, and the fact that they are two separate countries today is more an accident of history than anything deeper.
The only time that historic Germany has ever been a "relatively coherent entity" was in the Nazi period. (I think Hitler's role in Austria-Germany relations was sort of paradoxical, in that despite briefly unifying the two countries, he ended up associating the whole idea of unification with his own odiousness, such that Austria-Germany unification is a lot less likely now than it would have been had Hitler never happened.)
> The boundaries of Europe for some reason always seem to 'snap back' to something resembling an ethnographic map.
Yes and no. On the one hand, political boundaries often (but not always) end up following ethnic boundaries. On the other hand, ethnicity is often a deliberate political construction. Look at France – for many decades, even centuries, the policy of the French central government was to suppress regional ethnic/cultural/linguistic identities in favour of a homogenous notion of Frenchness. If people on opposite sides of France see themselves today as having a common ethnic identity, that is due to the deliberate policy of the French state, rather than something that happened naturally.