In the year 800, at the high altar of St. Peter's basilica in Rome, Pope Leo III crowned the Frankish King Charles as Emperor of Rome. When Charles the Great, better known as Charlemagne, accepted the crown from Leo III, he declared himself to be the legitimate heir to the throne of Constantine. The coronation of the emperor by the Pope strengthened both the Church and the state: the Pope reaffirmed his privilege to crown and anoint the ruler, and received military assistance in exchange; the new emperor could claim divine sanction for his acts and by this means gain moral and psychological superiority over his political foes. Theoretically, in 800 the Roman Christian Empire of Constantine was reestablished as people imagined it to have been. The new imperial realm, however, did not extend over Constantine's vast domain, for the Byzantine emperor or empress ruled the East from Constantinople. Furthermore, Charlemagne, with his interest and authority focused on the lands of France, Germany, and Italy, moved the political center of Western Europe from Rome to Aachen in Germany. Still, the dream of a unified, all‐ embracing European empire took hold of contemporary imagination, and, under the strong hand of Charlemagne, it became a near actuality.
How did a Frankish king, descended from Merovingian warlords, become the emperor of Western Europe? Charlemagne's unrivaled position had its roots in the instability of the Merovingian dynasty after the death of Clovis (511). The successors of the great sixth-century leader, challenged by intriguing, aggressive enemies—and further immobilized by their own sloth and incompetence—relegated more and more administrative duties to court officials. Consequently, the mayor of the palace, at first accountable only for the royal estates, assumed responsibility for the day-to-day management of the kingdom, its finances, and its army. By the time Pepin rose to the office at 697, the mayor of the palace was the virtual ruler of France, and the office had become a hereditary position. Because Pepin left no rightful heirs at his death in 714, the succession passed to his illegitimate son, Charles Martel ("The Hammer," 717-741). Charles not only brought the Merovingian nobility under his sway, but, by defeating the invading Islamic forces at the battle of Tours in 732, he also made Western Europe safe for Christianity.
Charles Martel's son, Pepin the Short (741‐ 768), finally deposed the last survivor of the Merovingian dynasty. To obtain sanction for his act, Pepin called upon the papacy, and in return for political assistance Pope Stephen II reanointed Pepin as King of the Franks in 754 at the Abbey of St. Denis, thus initiating the close association between the Frankish monarchs and the vicars of Christ. When Pepin died in 768, his sons, Carloman and Charles, divided the kingdom, but Carloman died in 771, thereby leaving Charles—Charlemagne— the sole monarch of the Franks.