The EU is what changed in Europe.
US "occupation" in Europe after WW2 was weak compared to the influence of the EU. The EU emanates all regulation in Europe. The US certainly didn't create the single market in the EU.
Also the brutal occupation of Easter Europe by the USSR is exactly an example of what the EU peace project is about solving.
The common market has probably been a net force for peace, but superpowers staking out areas where they were involved can hardly be brushed aside. After the fall of the USSR it hasn't been 30 years and we see land wars in their former sphere of influence.
Economics doesn't stop armies. We saw in WWII that insane politicians are perfectly capable of destroying everything in sight, economic consequences for their own citizens be damned.
I don't think it is the EU that is keeping the peace; I reakoon it is the Haber process + the US more than anything else. Although if it was it is failing IMO, there is a land war in Europe right now that is at risk of fighting WWIII. And everyone is arming up again because of it.