At the turn of the ninth century, the Roman (Byzantine) Empire was ruled by the young Emperor Constantine VI and his mother Eirene. In 797, Eirene assassinated her son and became the first empress regnant in Roman history. Three years later, with the position of emperor still vacant, Pope Leo III invited the Frankish king Charlemagne to celebrate Christmas in Rome. During the mass, the pope placed a crown on Charlemagne's head and hailed him as "Augustus of the Romans."
Over the next fourteen years, Emperor Charlemagne united most of western Europe under the Holy Roman Empire. However, this unity was short-lived. Charlemagne was succeeded by his son Louis, and upon his death, his three sons fought a civil war that resulted in the empire being split into three parts: West Francia, which eventually became the Kingdom of France; East Francia, which eventually became the Kingdom of Germany; and Middle Francia or Lotharingia, which France and Germany would spend the next thousand years fighting over.
While France remained united under a single king, the rest of the Holy Roman Empire became a famously disorganized confederation of kingdoms, principalities, bishoprics, counties, duchies, burgraviates, electorates, and free imperial cities. Some, such as Saxony, Brandenburg, Austria, and Bohemia, were secular kingdoms ruled by their own king, prince, duke, or other noble. Other territories, such as Cologne, Mainz, and Trier, were ecclesiastical (church) provinces ruled by a Prince-Archbishop. All of these were represented in the Imperial Diet and loosely united under an elected emperor. The organization of the Holy Roman Empire shifted continually but survived in some form for the next thousand years.
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