The unified Frankish empire under Charlemagne and his successors, the largest western Christian polity since Rome, dissolved by the Treaty of Verdun.
Fractured permanently in 888 following the deposition and death of Charles the Fat, the last ruler to briefly reunite all Carolingian kingdoms, with the nobility electing regional non-Carolingian kings.
Covers the Carolingian period as well as the Merovingian foundation — verified via Open Library.
Contemporary biography of Charlemagne by a member of his court; the foundational primary source for the Carolingian Empire at its height (primary source).
Primary sources: the official Royal Frankish Annals plus Nithard's eyewitness history of the wars among Louis the Pious's sons that led to the empire's partition (primary source).