That Europeans didn't fight in any of them. As the pop history has it...
"After two bloody wars causing untold misery and bloodshed, Europeans decided they had had enough of war and set out to build a strong and prosperous continent built on peace and harmony."
Except, of course, for all the nasty little conflicts in the post-colonial endgame, from Indochina to Ulster. Not to mention that the "peace" that was attained in Europe owed at least as much to the ongoing standoff between two heavily-armed nuclear powers, as to any newfound pacifistic tendencies among the Europeans themselves.
Plus, why would they need TWO wars to realize that war is so awful? "After deliberately smashing two of my fingers to bits with a hammer, I figured that smashing my fingers to bits with a hammer was pretty painful, and decided never to do it again."