Yugoslavia wasn't part of the EU, so it doesn't apply here. Fact is no EU country attacked another EU country since the EU (and its predecessors) came into existence. Before that time the same countries attacked each other on a fairly regular schedule.
It really doesn't do anyone any favours to make claims like this; the breakup of Yugoslavia and the associated genocides were hideous:
https://en.wikipedia.org/wiki/Breakup_of_Yugoslavia
(cards on table: I'm very pro-EU)