Before WWII, Europe was split into a series of multi-national alliances that were aimed against one another. After WWII, Western Europe was essentially united against the Soviet Union (and tied closely to the United States).
Before the war, there was the alliance between Germany and Italy on the one side and the alliance between France and Great Britain on the other. These alliances were bilateral agreements. After the war, instead of bilateral agreements there were multinational organizations created. The best example of this was NATO, which included most of the nations of Western Europe (along with the US) and was aimed at the Soviet Union. There was also the EU, which was also meant to bind all of the nations of Western Europe more tightly together in one democratic/capitalistic bloc.