something i find fascinatingly annoying is that in medieval Europe, various kings would carve their realm up. A well known example being the Franks under Charlemagne via the Treaty of Verdun. Now, i know that ethnonationalism wasn't exactly a thing at the time, but wouldn't it make more sense to give one son the whole kingdom, or simply make the other two dukes of smaller parts (like Bavaria and Acquiataine while the elder son ruled the rest)?
There's more to be said on the topic, especially giving the length (several centuries) and the area (different cultures and political traditions across Europe), but you might find some elements of answer about territorial partitions in Merovingian and Carolingian Francia there.