This increasing concentration of spiritual and political power in Rome was made possible by the absence of any alternative power structures, following the collapse of the central imperial administration in the late fifth century. Strong popes imposed their authority. Leo the Great, pope from 440–61, played a particularly important role in solidifying the author- ity of the papacy at both the theoretical and practical levels.
Leo introduced the use of the term pontifex maximus (the Roman term for the chief priest of the city in pagan times, and later transferred to the emperor) to refer to the pope. He also framed some of the traditional arguments that subsequently became normative for papal claims to authority. For example, Leo argued that Jesus of Nazareth had made Peter and his successors the rock on which his church would be built. Since the bishop of Rome was the successor of Peter, who had been martyred in the city, it followed that the pope was the ultimate foundation of the church.
Although the Roman Empire had collapsed in the west, the eastern empire was unaf- fected by the invasions in the west (1.4.7). During the sixth century, emperors based in Constantinople began a military campaign to recapture Italy, and incorporate it into the eastern empire. These campaigns were not totally successful. Nevertheless, by the seventh century, Byzantium had established authority over a large area of territory, which took the form of a diagonal band running roughly from Ravenna in the northeast of Italy to Rome and Naples in the southwest. Yet the emperor found it difficult to assert authority in the western area of Italy, allowing the pope to exercise considerable political and social influ- ence in these regions.