It's also why the EU was founded, and in that instance it worked great. European powers used to be constantly at war with each other, but in the last 70 years there was no large-scale war within Europe (except for Ukraine/Russia, both not in the EU), and war between EU states has become unthinkable.
This is surely a contributing factor, but being first-class citizens of Pax Americana US hegemony has been a larger one IMO (doubly so during the Cold War, when a common enemy on Western Europe's borders united them).
There's plenty of good things from a moral perspective about power being diffused away from a hyperpower hegemon, but stability and peace have never been among the side effects.
