What undid the work of Charlemagne in bringing peace to Europe?

In short, his descendants (grandchildren) fought over the royal line of succession splitting his once United Kingdom into three separate kingdoms. The West franks, the Central franks, and the East franks. His three grandchildren Lothair I, Louis the German, and Charles the Bald did this. After that their children fought over the royal line of succession, sometimes putting brother against brother or cousin against cousin-- This was actually common in those days. So in short, his heirs split up his kingdom, therefor split up the people and unity.