If anything, it's the post WW1 that was taken as a lesson: instead of further afflicting the defeated, fomenting nationalism and isolationism, a more conciliatory path was taken.

The union has its warts and boils, but the fact that the member nations have been at peace between themselves for the longest time span ever is hard to attribute to other factors.

They were not part of the EU. The reasoning is that if you form a union where there is a large amount of economical interdependence, you think twice about starting a conflict with other members of your union. The collapse of their economy means the collapse of your economy.