Boxer Rebellion, World War 1, etc.
There have been plenty of times that European countries have teamed up, and it wasn’t uncommon that sooner then later they would just start going to war with each other all over again.
What was it about WW2 and the Cold War that European Countries just went “aight, you’re cool let’s become friends and economically team up” and never went back to internal mayhem?
More can be said about Europe as a whole, but this answer deals with England and France Why did the French and English hate each other for around a millenia, but as soon as WWI and WWII roll around, they are suddently best buds? by u/crrpit