The Kalmar Union lasted from 1397 to 1523. Here is a wiki on it. Imagine Denmark, Norway, and Sweden united as a single country when it came to foreign affairs, but each of them having plenty of room to govern themselves domestically. The main rivalry here was the “monarchy” of Kalmar and the aristocracies of both Sweden and Denmark. This domestic rivalry, coupled with fact that its neighborhood included the Holy Roman Empire and the Hanseatic League, means that the Kalmar Union is probably one of the more interesting polities in European history. Yet I know next to nothing about it…