A strong external military force, along with the devastation of European countries with global ambitions in WWII (UK, French, Germany, and co), means there were neither resources nor reason for war in Europe for most of the 20th century. Heck, for most of that time Germany was still split in two.

USSRs occupation of the other half also kept "peace" there (and in conflict between the two sides would have escalated to a full global war, which was in nobody's interest).

Yugoslavia for example was only a state because the different nations were kept under the same rule. When that rule was gone, 60 years of "living together" didn't do anything to prevent ethnic tensions.

How things shape in 10 or 20 years is anybody's guess.