Charlemagne became king of the Franks in 768, initially sharing the kingdom with his brother Carloman. After Carloman's death in 771 he established sole rule. He intervened in Italy against the Lombards, captured Pavia in 774, and took the Lombard royal title. Further campaigns extended Frankish power into Bavaria and the Avar territories, while warfare and political bargaining drew aristocratic and ecclesiastical elites into his expanding realm.

The Saxon wars, beginning in 772, continued through repeated rebellions until 804. Conquest was bound to compulsory Christianization, the destruction of religious sites, punitive laws, and forced resettlement. The execution of captives at Verden in 782 became a particularly notorious episode, although historians debate the reported numbers. These campaigns imposed enormous suffering alongside the cultural achievements of his reign. In Spain, his expedition of 778 ended with the destruction of the rear guard at Roncesvalles by Basque attackers, later transformed in the Song of Roland.

Charlemagne supported Pope Leo III during a crisis in Rome. On Christmas Day in 800, Leo crowned him emperor. The title revived an imperial claim in western Europe and created difficult relations with the Byzantine rulers, who also claimed Roman authority. Charlemagne's power nevertheless continued to rest on his Frankish kingdom, military resources, and negotiations with the leading families of his territories.

Government relied on counts, bishops, royal assemblies, and itinerant representatives known as missi dominici. Written directives sought to regulate justice, religious practice, and the responsibilities of officials, although enforcement varied. Charlemagne promoted church reform and learning, gathering scholars including Alcuin of York. Schools, the correction of religious texts, and manuscript production contributed to the movement later called the Carolingian Renaissance. His palace and chapel at Aachen became centers of government and royal religious display.

In 813 he associated his surviving son Louis with the imperial office, and Louis succeeded him after his death in 814. The empire later divided among descendants, but its institutions and cultural activity influenced subsequent European rulers. Einhard's biography preserves testimony from his court in an admiring portrait requiring critical reading. Charlemagne's career combined conquest, Christian kingship, administrative reform, and sponsorship of learning.