Coronation
After Leo III became pope in 795, he faced political opposition. His enemies accused him of a number of crimes and physically attacked him in April 799, attempting to remove his eyes and tongue. Leo escaped and fled north to seek Charlemagne's help. Charlemagne continued his campaign against the Saxons before breaking off to meet Leo at Paderborn in September. Hearing evidence from the pope and his enemies, he sent Leo back to Rome with royal legates who were instructed to reinstate the pope and conduct a further investigation. In August 799 Charlemagne made plans to go to Rome after an extensive tour of his lands in Neustria. Charlemagne met Leo in November near Mentana at the 12th milestone outside Rome, the traditional location where Roman emperors began their formal entry into the city. Charlemagne presided over an assembly to hear the charges but believed that no one could sit in judgment of the pope. Leo swore an oath on 23 December, declaring his innocence of all charges. At mass in St. Peter's Basilica on Christmas Day 800, Leo proclaimed Charlemagne "emperor of the Romans" (Imperator Romanorum) and crowned him. Charlemagne was the first reigning emperor in the west since the deposition of Romulus Augustulus in 476. Charles the Younger was anointed king by Leo at the same time.
Historians differ about the intentions of the imperial coronation, the extent to which Charlemagne was aware of it or participated in its planning, and the significance of the events for those present and for Charlemagne's reign. Contemporary Frankish and papal sources differ in their emphasis on, and representation of, events. Einhard writes that Charlemagne would not have entered the church if he knew about the pope's plan; modern historians have regarded his report as truthful or rejected it as a literary device demonstrating Charlemagne's humility. Collins says that the actions surrounding the coronation indicate that it was planned by Charlemagne as early as his meeting with Leo in 799, and Fried writes that Charlemagne planned to adopt the title of emperor by 798 "at the latest." During the years before the coronation, Charlemagne's courtier Alcuin referred to his realm as an Imperium Christianum ("Christian Empire") in which "just as the inhabitants of the Roman Empire had been united by a common Roman citizenship", the new empire would be united by a common Christian faith. This is the view of Henri Pirenne, who says that "Charles was the Emperor of the ecclesia as the Pope conceived it, of the Roman Church, regarded as the universal Church".
The Eastern Roman (Byzantine) Empire remained a significant contemporary power in European politics for Leo and Charlemagne, especially in Italy. The Byzantines continued to hold a substantial portion of Italy, with their borders not far south of Rome. Empress Irene of Athens had seized the throne from her son Constantine VI in 797, deposing and blinding him. Irene, the first Byzantine empress, faced opposition in Constantinople because of her gender and her means of accession. One of the earliest narrative sources for the coronation, the Annals of Lorsch, present a female ruler in Constantinople as a vacancy in the imperial title which justified Leo's coronation of Charlemagne. Pirenne disagrees, saying the coronation "was not in any sense explained by the fact that at this moment a woman was reigning in Constantinople." Leo's main motivations may have been the desire to increase his standing after his political difficulties, placing himself as a power broker and securing Charlemagne as a powerful ally and protector. The Byzantine Empire's lack of ability to influence events in Italy and support the papacy were also important to Leo's position. According to the Royal Frankish Annals, Leo prostrated himself before Charlemagne after crowning him (an act of submission standard in Roman coronation rituals from the time of Diocletian). This account presents Leo not as Charlemagne's superior, but as the agent of the Roman people who acclaimed Charlemagne as emperor.
Historian Henry Mayr-Harting claims that the assumption of the imperial title by Charlemagne was an effort to incorporate the Saxons into the Frankish realm, since they did not have a native tradition of kingship. However, Costambeys et al. note in The Carolingian World "since Saxony had not been in the Roman empire it is hard to see on what basis an emperor would have been any more welcomed." These authors write that the decision to take the title of emperor was aimed at furthering Charlemagne's influence in Italy, as an appeal to traditional authority recognised by Italian elites within and (especially) outside his control.
Collins writes that becoming emperor gave Charlemagne "the right to try to impose his rule over the whole of [Italy]", considering this a motivation for the coronation. He notes the "element of political and military risk" inherent in the affair due to the opposition of the Byzantine Empire and potential opposition from the Frankish elite, as the imperial title could draw him further into Mediterranean politics. Collins sees several of Charlemagne's actions as attempts to ensure that his new title had a distinctly-Frankish context.
Charlemagne's coronation led to a centuries-long ideological conflict between his successors and Constantinople known as the problem of two emperors, which could be seen as a rejection or usurpation of the Byzantine emperors' claim to be the universal, preeminent rulers of Christendom. Historian James Muldoon writes that Charlemagne may have had a more limited view of his role, seeing the title as representing dominion over lands he already ruled. However, the title of emperor gave Charlemagne enhanced prestige and ideological authority. He immediately incorporated his new title into documents he issued, adopting the formula "Charles, most serene augustus, crowned by God, great peaceful emperor governing the Roman empire, and who is by the mercy of God king of the Franks and the Lombards" instead of the earlier form "Charles, by the grace of God king of the Franks and Lombards and patrician of the Romans." Leo acclaimed Charlemagne as "emperor of the Romans" during the coronation, but Charlemagne never used this title. The avoidance of the specific claim of being a "Roman emperor", as opposed to the more neutral "emperor governing the Roman empire", may have been to improve relations with the Byzantines. This formulation (with the continuation of his earlier royal titles) may also represent a view of his role as emperor as being the ruler of the people of the city of Rome, as he was of the Franks and the Lombards.