"No more brother wars" is a relatively common saying in these political circles nowadays, but how would you propose to actually stop them from happening again in the future? Some of the nationalist ideals here make violent clashes inevitable

I have seen comments here that support violent irredentism, the most common example being retaking the old Prussian lands from Poland and Russia, but there are plenty others (e.g. Portuguese nats wanting Galicia added to Portugal).

Assume we live in your ideal world, the immigrants are gone, Europe is ruled by fascist nationalists from the Urals to the Azores. What will be done to stop the mistakes from 1914 and 1939 from happening all over again, and killing millions more Europeans again? The ideology seems far too prone to in fighting once the common enemy is vanquished.