After the fall of the (West) Roman Empire, countries in West Europe had wars many times until the end of WW2. In this situation, I think European Coal and Steel Community, the former union of EU, played the important role in keeping peace all over European countries.
But, currency union caused the debt crisis in some countries. I guess this was one of the failures EU led.
Even after wining the supreme prize, it is necessary for EU to pay attention to enforcing policies.