That is a good thing.
Nationalism has interesting "properties".
It carries with it the implicit assumption that people miles away in the same nation that you have never met share some of the same beliefs and values you do.
It somehow justify you taking credit for the accomplishments of others in the same nation even though you haven't done anything to contribute to said accomplishment.
You may be objectively correct, but if it weren't for nationalism, my culture wouldn't exist today and I would have been a German, as would have been my ancestors for more than a century. Just because something is "artificial and pointless" doesn't mean that it's bad.
I wouldn't be so sure about that. Frequently the conqueror gets culturally influenced by the people they conquer - especially 100+ years ago where totalitarian levels of control were much harder to implement.
Also when your people repelled the Germans, it might have more to do with cultural differences than nationalism - empires who succeed in holding on to capture territory are often those that absorb and incorporate the culture of the people they conquered.
> empires who succeed in holding on to capture territory are often those that absorb and incorporate the culture of the people they conquered
Well I didn't notice a lot of Germans and Austrians speaking Czech, to be honest.
"identification with one's own nation and support for its interests, especially to the exclusion or detriment of the interests of other nations."