Peace in Europe is a bit of an outlier [1]. The period you refer to is also not a good example given Europe’s peace since WW2 has been more a product of American support and might than European identity.

I think Europe was far more ethnically homogenous before WW2 and as the link shows rather violent. So your reasoning is just plain wrong.

[1] https://en.m.wikipedia.org/wiki/List_of_conflicts_in_Europe