The 28 countries of the EU have seen no military engagements between member states. Some of these countries - Britain, Germany, France, Spain, Holland, Italy have a history of a constant flux of shifting multiple alliances and conflicts with other alliances. Below is a table of wars just for 100 years before the EU, but of course a thousand years of European history is like this.
"Ah, but that's just NATO. Its NATO that keeps the peace." And of course NATO is very important in keeping the peace between Western European nations. But current NATO leaders make it clear that the EU is an important partner organisation (see below). And when Nations trade so closely together, and have a parliament together, the pressures that lead to war are very unlikely to happen.
|1939-1945||World War II|
|1936-1939||Spanish Civil War|
|1921-22||Irish War of Independence|
|1914-1918||World War I|