The most illustrious ruler of the middle ages, Charlemagne (2 April 742 – 28 January 814) was crowned Emperor of the Romans by Pope Leo III on Christmas Day at Rome's Old Street, Peter's Basilica. During a more tedious power struggle with his brother than Pepin his father endured, Charlemagne sort marriage with King Desiderius’s daughter (of the Lombard’s), to affix power in furthering peace for the Papacy’s territory. Though this was against the mandate of Pope Stephan III, as Charles was already married to Himiltrude. Pepin’s previous grant to the Papacy of the territory of central Italy, was in jeopardy however still, and due to the eastern powers centred in Constantinople. When Charlemagne’s father, Pepin had welcomed Pope Stephan to the Carolingian Royal Palace at Ponthion in 754, Pepin had acted to restore Papal lands taken by Aistulf in central Italy. Aistulf, the military ruler of the Lomabards had seized the imperial capital of Ravenna from the Papacy. In 755 and 756 Pepin, Charlemagne’s father entered Italy to vindicate Pope Stephan, and defeated Aistulf in the Alps, after an attempted siege on Rome. A mandate for peace was attained after Frankish forces pursued Aistulf’s army to and plundered his land around Pavia. Yet Aistulf proved ever troublesome continuing to taunt the Papacy with a second attempt on Rome also quelled again by the Franks in 756. The keys to a number of cities and territories in central Italy that had submitted to Papal authority were collected duly with the list of the cities involved, as described in the Confession of St. Peter.
Charlemagne’s primary military activity against the Saxons would compel him for 30 years, after which achieving the annexation of a large block of territory between the Rhine and the Elbe rivers. The period is stained by pillaging, broken truces, hostage taking, mass killings, deportation of rebellious Saxons, draconian measures to compel acceptance of Christianity, and occasional Frankish defeats. The Frisians, Saxon allies living along the North Sea east of the Rhine, were also forced into submission in turn. During this time Charlemagne attained sole authority over the Franks, with the death of his own brother and co-ruler of Francia, Carloman. The authority of the Holy See, in according Monarchy, and Ducal powers was contested however and crisis ensued after the death of Pope Paul I in 767. Desiderius had seized a priest named Philip from the Monastery of St. Vitus on the Esquiline Hill in Rome on Sunday, July 31, 768, and summarily appointed him Pope. This Antipope Philip was never recognised nor gained a significant following, thus returning to the monastery where he was never heard from or seen again, leaving the Papacy to appoint Pope Adrian I without contestation.
Afterwards (See Chronology of Carloman I) Charlemagne repudiated his wife and King Desiderius’s daughter, and married a beautiful Swabian. Furious, Desiderius's Lombard’s again marched on Rome after attacking Pope Adrian first and invading the Pentapolis. The embassies of Adrian and Desiderius met at Thionville and Charlemagne favoured the Pope; marching on the Lombard capital of Ticinum with his full Frankish force. Desiderius' son Adelchis was raising an army at Verona meanwhile, but the young prince fled to Constantinople to escape Charlemagne. The siege on Pavia was absolved when, in return for the lives of his soldiers and subjects, Desiderius surrendered and opened the gates, henceforth sent to exile at Corbie Abbey. On entry to Rome, Charlemagne had, and after a series of consolidated campaigns such as against the Lombard duchy of Benevento in southern Italy; defined the territory of Italy. Installing his second son to wife Hildegard, King Pippin from 781–810. Pippin would in turn vitally securitise relations with the Byzantines.
Charlemagne’s relations with the papacy, especially with Pope Adrian I, were good and brought him valuable support for his religious program and praise for his qualities as a Christian leader. The expanded Frankish presence in Italy and the Balkans intensified diplomatic encounters with the Eastern emperors, which strengthened the Frankish position with respect to the Eastern Roman Empire, weakened by internal dissension and threats by Muslims and Bulgars on its eastern and northern frontiers. Charlemagne also established friendly relations with the ʿAbbāsid caliph in Baghdad (Hārūn al-Rashīd), the Anglo-Saxon kings of Mercia and Northumbria, and the ruler of the Christian kingdom of Asturias in northwestern Spain. Charlemagne obtained the role as protector of Jerusalem furthermore. The Eastern Orthodox Church however would come to consider Charlemagne a heterodox for supporting the filioque (concerns generational advance of the Holy Spirit). Their disdain likewise was in recognition by the Bishop of Rome as a legitimate Roman Emperor, rather than the ‘basilissa’, Irene of Athens of the Eastern Roman Empire. These and more machinations led to the split of Rome and Constantinople in the Great Schism of 1054.