Gregory II (715–731) marks a transition to a new state of things. He clashed with the emperor Leo the Isaurian about the worship of images. Under his pontificate Liutprand, the mightiest king of the Lombards, conquered Ravenna and became ruler of Italy. But the popes found the dominion of a barbarian and formerly Arian authority more loathsome to the than that of distant Constantinople. So, stuck between a heretical emperor and a barbarian, they looked to the young and rising power of the Franks for deliverance and protection. The Franks had been converted under Clovis and under Charles Martel (the Hammer) had attained an impressive victory over the Saracens (732), which saved Christian Europe against the oppression of Islam. They had therefore become the protectors of Latin Christianity. They also lent their aid to Boniface in the conversion of Germany.
Pope Zacharias (741–752) brought Liutprand to terms of temporary obedience. Astolph, his successor once again threatened to integrate Rome with his kingdom. Zacharias sought the protection of Charles Martel’s son and Charlemagne’s father, Pepin the Short. In return for this aid Zacharias helped him to the crown of France. This was the first step towards the creation of a Western empire and a new political system of Europe with the pope and the German emperor at the head.
In 800, while Charlemagne was kneeling at the alter in St. Peter’s celebrating Christmas, the pope, under a sudden inspiration (but no doubt because of a premeditated plan, placed a golden crown on his head and the people of Rome shouted, “To Charles Augustus, crowned by God, the great and pacific emperor of the Romans, life and victory!" From then on he was adored by the pope and was titled Emperor and Augustus instead of Patrician. For the West this was a reestablishment of the old Roman Empire. Together with the papacy the two controlled the history of the Middle Ages. The pope and the emperor represented the highest dignity and power in church and state.
The legal relation between the two powers is the struggle of centuries. In theory, it was easy to distinguish the two. The pope was confined to spiritual affairs and the emperor to temporal affairs. But because of their theocratic theory of the union of church and state the two quarreled frequently. Since the pope had voluntarily conferred the crown upon Charlemagne, he might claim that the empire was his gift, and that the right of crowning implied the right of un-crowning. This right was exercised by popes at a later period - popes who wielded the secular as well as the spiritual sword and absolved nations of their oath of allegiance. This is the medieval hierarchical theory, which derives all power from God through Peter as the head of the church.
Charlemagne, though he was devotedly attached to the church and the pope, was too absolute a monarch to recognize any other sovereignty within his own. His idea of theocracy was derived from the Old Testament relation between Moses and Aaron. He really believed that he was the divinely appointed protector of the church and the regulator of all her external (and to some extent also her internal) affairs.
Charlemagne‘s main ambition was to unite all the Teutonic and Latin races on the Continent under his temporal scepter in close union with the spiritual dominion of the pope. His ecclesiastical domain extended over twenty-two archbishoprics or metropolitan sees. He gave his personal attention to things great and small. He introduced a settled order to his empire. He secured Europe against future heathen and Muslim invasion. He was universally admired or feared in his age.
Charlemagne was a firm believer in Christianity and a regular worshipper in the church. He is recorded to have attended church going morning and evening, even after nightfall, besides attending the regularly scheduled mass. He was very generous to the clergy. He gave them tithes throughout the empire. He appointed worthy bishops and abbots, endowed churches and built the grand cathedral at Aix-la-Chapelle, where he was buried. His respect for the clergy went so far as the veneration of the bishop of Rome as St. Peter’s successor.
In spite of his many virtues, he was not as pure as the poetry of the church represented him, and far from being worthy of canonization. He sacrificed thousands of human beings to his immense ambition and obsession for conquest. He converted nations at sword-point in a war of religion for the annihilation of heathenism. This is contrary to the spirit of Christianity and is rather the Islamic principle: submission to the faith, or death. The most serious defect in his private character was his lack of self-control and disregard for the inviolability of the marriage bond. In this regard he was no better than an Oriental despot or a Muslim Caliph. He married several wives and divorced them at his pleasure.
Charlemagne died after a short illness on January 28, 814, at the age of 71 and in the 47th year of his reign. He was buried on the same day in the cathedral of Aix-la-Chapelle.