In 778 he invaded Spain, hoping to take advantage of civil war among the Muslim rulers of that kingdom, but was repulsed at Zaragoza. In later campaigns conducted by local counts, Barcelona was captured (801) and a frontier established beyond the Pyrenees. Charlemagne's struggle with the pagan Saxons, whose greatest leader was Widukind, lasted from 772 until 804. By dint of forced conversions, wholesale massacres, and the transportation of thousands of Saxons to the interior of the Frankish kingdom, Charlemagne made his domination over Saxony complete. In 788 he annexed the semi-independent duchy of Bavaria, after deposing its duke, Tassilo. He also warred successfully against the Avars and the Slavs, establishing a frontier south of the Danube.

In 799 the new pope, Leo III, threatened with deposition by the Romans, appealed to Charlemagne. Charlemagne hastened to Rome to support Leo, and on Christmas Day, 800, was crowned emperor by the pope. His coronation legitimized Charlemagne's rule over the former Roman empire in W Europe and finalized the split between the Byzantine and Roman empires. After years of negotiation and war, Charlemagne received recognition from the Byzantine emperor Michael I in 812; in return Charlemagne renounced his claims to Istria, Venice, and Dalmatia, which he had held briefly. The end of Charlemagne's reign was troubled by the raids of Norse and Danes (see Norsemen), so Charlemagne took vigorous measures for the construction of a fleet, which his successors neglected. His land frontiers he had already protected by the creation of marches. In 813, Charlemagne designated his son Louis I as co-emperor and his successor and crowned him at Aachen.

In his government Charlemagne continued and systematized the administrative machinery of his predecessors. He permitted conquered peoples to retain their own laws, which he codified when possible, and he issued many capitularies (gathered in the Monumenta Germaniae historica). A noteworthy achievement was the creation of a system by which he could supervise his administrators in even the most distant lands; his missi dominici were personal representatives with wide powers who regularly inspected their assigned districts. He strove to educate the clergy and exercised more direct control over the appointment of bishops and he acted as arbiter in theological disputes by summoning councils, notably that at Frankfurt (794), where adoptionism was rejected and some of the decrees of the Second Council of Nicaea (see Nicaea, Second Council of) were condemned. He stimulated foreign trade and entertained friendly relations with England and with Harun ar-Rashid. In 813, Charlemagne designated his son Louis I as co-emperor and his successor and crowned him at Aachen.

Charlemagne's court at Aachen was the center of an intellectual renaissance. The palace school, under the leadership of Alcuin, became famous; numerous schools for children of all classes were also established throughout the empire during Charlemagne's reign. The preservation of classical literature was aided by his initiatives. Prominent figures of the Carolingian renaissance included Paul the Deacon and Einhard.

In his daily life Charlemagne affected the simple manners of his Frankish forebears, wore Frankish clothes, and led a frugal existence. He was beatified after his death and in some churches has been honored as a saint. Surrounded by his legendary 12 paladins, he became the central figure of a cycle of romance. At first, legend pictured him as the champion of Christendom; later he appeared as a vacillating old man, almost a comic figure. His characterization in the Chanson de Roland (see Roland) has impressed itself indelibly on the imagination of the Western world. The vogue of the Charlemagne epic ebbed somewhat after the Renaissance but was revived again in the 19th cent. by Victor Hugo and other members of the Romantic school. Charlemagne's creation (or re-creation) of an empire was the basis of the theory of the Holy Roman Empire; it was his example that Napoleon I had in mind when he tried to assume his succession in 1804.

Einhard wrote a contemporary biography of Charlemagne. See also H. Fichtenau, The Carolingian Empire (1949, tr. 1957); D. Bullough, The Age of Charlemagne (1966); J. Boussard, The Civilization of Charlemagne (tr. 1968). For the literary aspect, see J. L. Weston, The Romance Cycle of Charlemagne and His Peers (1901).