Pope Leo III (died June 12, 816) was Pope from 795 to 816. Protected by Charlemagne from his enemies in Rome, he subsequently strengthened Charlemagne's position by crowning him as Holy Roman Emperor.
Leo, who came from a common-folk background, had risen in the hierarchy of Rome and was elected Pope only one day after the death of his predecessor, Pope Adrian I, who had worked for good relations between Rome and the Frankish Empire under Charlemagne. Leo announced his election to Charlemagne, sending him the keys of Saint Peter's tomb and the banner of Rome, requesting an envoy. Charlemagne in his reply stated that it was his function to defend the church, and the popes' to pray for the realm and for victory of the army.
Leo aroused the hostility of Rome's nobility, who saw the papal post as reserved for noble candidates. During his rule he was accused of adultery and perjury. In April 799 he was attacked by a gang, who unsuccessfully attempted to gouge out his eyes and cut off his tongue for his earlier actions, only to be saved by Magnus Forteman and 700 Frisian nobles of his army. He was then formally deposed and sent to a monastery, but escaped and made his way to Paderborn, where he met Charlemagne.
Charlemagne ordered Leo's enemies to Paderborn, but no decision could be found. He then had Leo escorted back to Rome. In November 800 Charlemagne himself went to Rome, and on December 1 held a council there with representatives of both sides. Leo, on December 23, took an oath of purgation concerning the charges brought against him, and his opponents were exiled.
Two days later, on Christmas Day 800, Leo crowned Charlemagne as emperor, thus creating the status of Holy Roman Emperor. This offended Constantinople, which had traditionally been seen as the defender of Rome, but the Byzantine Empress Irene of Athens was too weak to oppose Charlemagne. Charlemagne was to intervene in church affairs, not always being successful.
Leo helped restore King Eardwulf of Northumbria, and settled various matters of dispute between the Archbishops of York and Canterbury. He also reversed the decision of his predecessor, Pope Adrian I, in regards to the granting of the pallium to the bishop of Lichfield, Higbert. He believed that the English episcopate had been misrepresented before Adrian and that therefore his act was invalid. In 803, Lichfield was a regular diocese again.
Leo forbade the addition of "filioque" to the Nicene Creed which was added by Franks in Aachen in 809. He also ordered that the Nicene creed be engraved on silver tablets so that his conclusion might not be overturned in the future. He wrote «HAEC LEO POSUI AMORE ET CAUTELA ORTHODOXAE FIDEI» (I, Leo, put here for love and protection of orthodox faith)(VITA LEONIS, LIBER PONTIFICALIS (Ed.Duchene, TII, p.26)
The reasons for the coronation of Charlemagne, the involvement beforehand of the Frankish court, and the relationship to the Byzantine Empire are all matters of debate among historians. An effective administrator of the papal territories, Leo contributed to the beautification of Rome.