Your challenge, should you choose to accept it, is to create a union of European states at the earliest possible moment you can feasibly manage.
1. It has to be an amalgation of states that truly (and not just hypothetically) respect each other as sovereign equals, even if some of them are obviously more powerful than others (just like France and Germany are more powerful than, say, the Netherlands or Luxembourg in the modern EU). It must not be one powerful state that has basically reduced other states to vassal status.
2. It has to cover a substantial part of Europe. Doesn't have to be all of Europe, but the German Confederation or a hypothetical United States of Austria, for instance, do not qualify. (In antiquity, a union of various states surrounding the Med would be acceptable, since including northen Europe back in those days would be tricky.)
3. It cannot just be a temporary alliance. It has to be an intended-to-be-permanent league.
Anyway, have at it!