I'd argue we didn't see a war between Germany and England, or Britain as we are these days, as we were occupying them in the aftermath until 1949 and maintained

I'd argue we didn't see a war between Germany and England, or Britain as we are these days, as we were occupying them in the aftermath until 1949 and maintained a British Army of the Rhine there until 1994. The Allies also established a new Constitution and political consensus that is vehemently anti-war - witness German reluctance to be involved in fighting in recent NATO and other coalition conflicts. There's also the small matter that the Russians were effectively still occupying half of Germany until the late 1980s. The origins of the EU came that trade prevents wars. The Democratic Peace Thesis likes to discount all sorts of countries as "not a democracy", a good example of No True Scotsman. Given we're seeing Turkey rapidly heading towards dictatorship, Hungary's far right leadership despite being an EU member, and all the problems the US is going through it is a salient reminder that just because you are a democracy doesn't mean you stay one. One of the pretty unique things about the UK in the EU is its political history - France is on its 5th Republic, Germany has only been reunified within living memory, Italy's system is from 1946 etc. You can see how relatively new, potentially fragile systems of Government might want to prevent resurgences of problems, especially under the shadow of the Cold War. Edit to add: Oh - there's also this thing called NATO as well. When two countries are in a military alliance they also tend to avoid going to war with each other. NATO is of course also the body that intervened far more decisively than the EU in the Bosnian and Kosovan Wars - those pesky examples of genocide in Europe that people overlook about how fantastic and peaceful we are as a continent.