The purpose of the EU is to prevent war. The EU hasn't had a war in 75 years. That is the longest that Europe hasn't been at war in its recorded history.
Also, I understand that Russia is considered less and less "European" with each passing year but the Ukraine and Russia (or at least all of its centers of power) are located within the European continent by definition. There's also the issue of the IRA's war against Northern Ireland and the UK as a whole, while that conflict doesn't meet the commonly agreed definition of "traditional warfare", it was the very template every modern insurgency bases itself upon. In a sense that was a war by any modern definition since traditional warfare seems to have become a relic of the past.
The EU was a necessary prerequisite of the peace process in northern Ireland, as is being brought into sharp focus now With Brexit.
The EU is if anything a force against peace, as it takes a totalitarian "you obey us on everything or we work with you on nothing" approach which may appear to create temporary peace, but only by suppressing dissent. The USSR was also by this approach very "peaceful".