The EU has actually probably stopped wars, but probably not wars that would have been waged between its members, but wars between non-EU and EU countries. Just look at Cyprus, which was extremely precarious before it joined.
The EU has done lots of good stuff actually, like useful consumer protection laws, environmental programs, research, infrastructure and growth grants, beneficial free trade and travel between members, a strong common currency with low inflation, etc. However it's also responsible for lots of very awful stuff (some of them being the other side of the coin to the positive stuff mentioned before) which is why some people want out and some want reform. Widespread radical reform is the best option but it is really hard to achieve that, so we're in a bit of a pickle.