Coronation

In 799, after Pope Leo III was abused by Romans who tried to put out his eyes and tear out his tongue, he escaped and fled to Charlemagne at Paderborn. Charlemagne, advised by scholar Alcuin of York, travelled to Rome in November 800 and held a council on December 1. On December 23, Leo swore an oath of innocence. At Mass, on Christmas Day (December 25), when Charlemagne knelt at the altar to pray, the pope crowned him Imperator Romanorum ("Emperor of the Romans") in Saint Peter's Basilica. In so doing, the pope effectively nullified the legitimacy of Empress Irene of Constantinople. As historian James Bryce writes: 

When Odoacer compelled the abdication of Romulus Augustulus, he did not abolish the Western Empire as a separate power, but caused it to be reunited with or sink into the Eastern, so that from that time there was a single undivided Roman Empire ... [Pope Leo III and Charlemagne], like their predecessors, held the Roman Empire to be one and indivisible, and proposed by the coronation of [Charlemagne] not to proclaim a severance of the East and West.

Charlemagne's coronation as emperor, though intended to represent the continuation of the unbroken line of emperors from Augustus to Constantine VI, had the effect of setting up two separate (and often opposing) empires and two separate claims to imperial authority. For centuries to come, the emperors of both West and East would make competing claims of sovereignty over the whole. 

In support of Charlemagne's coronation, some argued that the imperial position had actually been vacant, deeming a woman (Irene) unfit to be emperor. However, Charlemagne made no claim to the Byzantine Empire. Whether he actually desired a coronation at all remains controversial—his biographer Einhard related that Charlemagne had been surprised by the pope. Regardless, Byzantium felt its role as the sole heir of the Roman Empire threatened and began to emphasize its superiority and its Roman identity. Relations between the two empires remained difficult. Irene is said to have sought a marriage alliance between herself and Charlemagne, but according to Theophanes the Confessor, who alone mentions it, the scheme was frustrated by Aetios, one of her favorite advisors.

Coronation of Charlemagne

The Coronation of Charlemagne, by assistants of Raphael, c. 1516–1517.

Motivation

For both the pope and Charlemagne, the Roman Empire remained a significant power in European politics at this time, and continued to hold a substantial portion of Italy, with borders not far south of the city of Rome itself. This is the empire that historiography has been labelled the Byzantine Empire, for its capital was Constantinople (ancient Byzantium) and its people and rulers were Greek; it was a thoroughly Hellenic state. Indeed, Charlemagne was usurping the prerogatives of the Roman emperor in Constantinople simply by sitting in judgement over the pope in the first place. Historian John Julius Norwich writes of their motivation:

By whom, however, could he [the Pope] be tried? In normal circumstances the only conceivable answer to that question would have been the Emperor at Constantinople; but the imperial throne was at this moment occupied by Irene. That the Empress was notorious for having blinded and murdered her own son was, in the minds of both Leo and Charles, almost immaterial: it was enough that she was a woman. The female sex was known to be incapable of governing, and by the old Salic tradition was debarred from doing so. As far as Western Europe was concerned, the Throne of the Emperors was vacant: Irene's claim to it was merely an additional proof, if any were needed, of the degradation into which the so-called Roman Empire had fallen.

For the pope, then, there was "no living Emperor at the that time." Furthermore, the papacy had since 727 been in conflict with Irene's predecessors in Constantinople over a number of issues, chiefly the continued Byzantine adherence to the doctrine of iconoclasm, the destruction of Christian images. From 750, the secular power of the Byzantine Empire in central Italy had been nullified.

Norwich explains that by bestowing the imperial crown upon Charlemagne, the pope arrogated to himself "the right to appoint the Emperor of the Romans, establishing the imperial crown as his own personal gift but simultaneously granting himself implicit superiority over the Emperor whom he had created." And "because the Byzantines had proved so unsatisfactory from every point of view—political, military and doctrinal—he would select a westerner: the one man who by his wisdom and statesmanship and the vastness of his dominions stood out head and shoulders above his contemporaries."

How realistic either Charlemagne or the pope felt it to be that the people of Constantinople would ever accept the king of the Franks as their emperor, we cannot know; Alcuin speaks hopefully in his letters of an Imperium Christianum ("Christian Empire"), wherein, "just as the inhabitants of the [Roman Empire] had been united by a common Roman citizenship," presumably this new empire would be united by a common Christian faith.

Roman Emperor

In any event, Charlemagne used these circumstances to claim that he was the renewer of the Roman Empire, which was perceived to have fallen into degradation under the Byzantines. The title of Emperor remained in the Carolingian family for years to come, but divisions of territory and in-fighting over supremacy of the Frankish state weakened its power and ability to lead. The papacy itself never forgot the title nor abandoned the right to bestow it. When the family of Charlemagne ceased to produce worthy heirs, the pope gladly crowned whichever Italian magnate could best protect him from his local enemies. This devolution led to the dormancy of the title from 924 to 962. The title was revived when Otto I was crowned emperor in 962, fashioning himself as the successor of Charlemagne. The empire would remain in continuous existence for nearly a millennium, as the Holy Roman Empire, a true imperial successor to Charlemagne.