It's putting kindle on the fire. If you keep telling people to do what's best for their nation - and further, if there's some confusion on what their nation exactly is - it's just a matter of time before someone decides to light the powder keg.
Nationalism for France and nationalism for Germany is one thing when you live in Paris or Berlin, but if you live in Alsace or Lorraine, suddenly you're faced with a very sticky option: either accept that who your nation is has been decided for you, or risk your life deciding that it's your mission to change it in the presence of a militaristic state.
What do you do if you live in Warsaw? Nationalism tells you that Poland was a flipping huge empire that proudly stood against the Mongols themselves, but now you've got to fight the Russians AND the Germans AND the Austrians, all of whom are ready to send in soldiers to shoot activists who dare suggest such a thing.
British nationalism and German nationalism motivated an arms race of every bigger, ever heavier weapons, and an itching curiosity to see what they would actually do in a combat scenario.
And that's not even getting past 1912.
Nationalism as the basis of claiming statehood unequivocally led to an unbridled mess. Twice. And to try to divorce it from nationalism makes no sense at all.