re: WW 2 without question !
After WW2, the UN was created, principles such as non interference of sovereign states were brought forward.
But especially: the longest period of peace in Europe (62 years and counting).
AND, the former endless enemies decided to get together and build a common market !!
The relationship between those countries is so strong it's become virtually impossible to wage wars against each other. Besided, our armies have been quite reduced! (Only professionnals, no more draftees)
In the Balkans, old rivalries never quite stopped, once Tito died, Serbs Croats and Bosniacs fought each other again. If they want to join the EU, they will have to forget their ancestral rivalries!