I know there has been intra-nation peace in western Europe since the end of WWII, but how do we know it was due to the EU?
Both the peace and the EU would seem to as easily explained by the post war situation where European nations were now second tier economicly, and threatened by a big external nation. The reward for war was no longer there, and the need for cooperation was high.
All of your points also existed before the second world war. Russia was already a big empire, other big empires existed throughout the history of Europe. That never stopped European countries from attacking each other. Basically, almost everything else has staid the same, the only change was the EU and its predecessors came into existence after WW2. That's certainly not a proof (it's impossible to proof such thins without a alternative-history-viewer-machine, which hasn't been developed so far) but it is a very strong hint.
I don't think either one of those was true after after WW1.
Russia, though large, was not regarded as a serious military threat to Europe. When attacked by Russia, Poland (!) beat Russia in the 1920's and gained half of the Ukraine and half of Belarus in the subsequent peace treaty.
An although a European nation was no longer the world largest economy, they were still regarded as the powers that mattered. The Washington Naval Treaty on arms control between the wars even gave the US a ship allocation on par with a European nation.
The world at the time still revolved around Europe and the actions of European powers. To be premininet in Europe was to lead the world.
This has been the longest period of Peace in Western Europe in history by a massive margin. That's evidence. Non top tier countries used to fight all the time in Europe before.
I don't think either one was true the case after after WW1.
Russia, though large, was not regarded as a serious military threat to Europe. When attacked by Russia, Poland (!) beat the snot out of Russia in the 1920's and gained half of the Ukraine and half of Belarus in the subsequent peace treaty.
An although a European nation was no longer the world largest economy, they were still regarded as the ones that mattered. The world at the time still revolved around Europe and the actions of European powers. To be premininet in Europe was to lead the world.
Both the peace and the EU would seem to as easily explained by the post war situation where European nations were now second tier economicly, and threatened by a big external nation. The reward for war was no longer there, and the need for cooperation was high.