Charlemagne was the first Holy Roman Emperor. This title symbolized the cooperation between church and state that ensured the Roman popes' authority over the Western Church, and the Frankish emperors' authority over much of Christianized Western Europe. Under Charlemagne, King of the Franks, the influence of the Catholic Church had been strongly reinforced. A new relationship between the Frankish kings and the popes was started by Pepin, Charlemagne's father, and Boniface, the popes' legate. Their early program of church reform was greatly expanded by Charlemagne. Pepin had also given his protection to the popes when Rome was threatened by invaders, and Charlemagne continued the tradition. This situation resulted in a new interdependence between church and state. Charlemagne exerted great influence on the clergy and on church practice, and offered security to the papacy. At the same time, the popes' blessing legitimized the Carolingians' right to replace the Merovingian kings, and their authority to govern a large empire. The relationship culminated in Pope Leo III's coronation of Charlemagne as Holy Roman Emperor in 800 A.D. In their conception of the Holy Roman Empire, church and state interests were combined in a way that was meant to provide mutual support. This was one of the great political and ecclesiastical innovations of the early Middle Ages.
Since the time of Constantine, the first Christian emperor of Rome, the Church had the protection of the Roman emperors. But, after the Roman emperors moved their capital to Constantinople, or Byzantium, the Eastern emperors were too far away or too weak, to uphold the Roman popes' "claim to 'universal' jurisdiction over Christendom." The popes remained the emperors' subjects, but did not receive the benefit of their protection. Other Catholic rulers were needed to reinforce papal authority in Western Europe. The Merovingian rulers of the Franks were Catholics, but,...