The first three decades of Charlemagne’s reign were dominated by military campaigns, which were prompted by a variety of factors: the need to defend his realm against external foes and internal separatists, a desire for conquest and booty, a keen sense of opportunities offered by changing power relationships, and an urge to spread Christianity. His performance on the battlefield earned him fame as a warrior king in the Frankish tradition, one who would make the Franks a force in the world once contained in the Roman Empire.

Charlemagne’s most demanding military undertaking pitted him against the Saxons, longtime adversaries of the Franks whose conquest required more than 30 years of campaigning (772 to 804). This long struggle, which led to the annexation of a large block of territory between the Rhine and the Elbe rivers, was marked 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.

While the conquest of Saxony was in progress, Charlemagne undertook other campaigns. As soon as he became sole king in 771, he repudiated his Lombard wife and his alliance with her father, King Desiderius. Soon after, in 773–774, he answered the appeals of Pope Adrian I (772–795) for protection by leading a victorious expedition into Italy, which ended with his assumption of the Lombard crown and the annexation of northern Italy. During this campaign Charlemagne went to Rome to reaffirm the Frankish protectorate over the papacy and to confirm papal rights to the territories conceded by Charlemagne’s father. Additional campaigns were required to incorporate the Lombard kingdom fully into the Frankish realm, however; an important step in that process came in 781, when Charlemagne created a subkingdom of Italy with his son Pippin as king.

Concerned with defending southern Gaul from Muslim attacks and beguiled by promises of help from local Muslim leaders in northern Spain who sought to escape the authority of the Umayyad ruler of Cordoba, Charlemagne invaded Spain in 778. That ill-considered venture ended in a disastrous defeat of the retreating Frankish army by Gascon (Basque) forces, immortalized three centuries later in the epic poem The Song of Roland. Despite this setback, Charlemagne persisted in his effort to make the frontier in Spain more secure. In 781 he created a subkingdom of Aquitaine with his son Louis as king. From that base Frankish forces mounted a series of campaigns that eventually established Frankish control over the Spanish March, the territory lying between the Pyrenees and the Ebro River.

In 787–788 Charlemagne forcibly annexed Bavaria, whose leaders had long resisted Frankish overlordship. That victory brought the Franks face to face with the Avars, Asiatic nomads who during the late 6th and 7th centuries had formed an extensive empire largely inhabited by conquered Slavs living on both sides of the Danube. By the 8th century Avar power was in decline, and successful Frankish campaigns in 791, 795, and 796 hastened the disintegration of that empire. Charlemagne captured a huge store of booty, claimed a block of territory south of the Danube in Carinthia and Pannonia, and opened a missionary field that led to the conversion of the Avars and their former Slavic subjects to Christianity.

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Charlemagne’s military successes resulted in an ever-lengthening frontier, which needed to be defended. Through a combination of military force and diplomacy he established relatively stable relations with a variety of potentially dangerous enemies, including the Danish kingdom, several Slavic tribes inhabiting the territory along the eastern frontier stretching from the Baltic Sea to the Balkans, the Lombard duchy of Benevento in southern Italy, the Muslims in Spain, and the Gascons and the Bretons in Gaul. The Italian scene was complicated by the Papal States, whose boundaries remained problematic and whose leader, the pope, had no clearly defined political status relative to his Frankish protector, now his neighbour as king of the Lombards. In general, Charlemagne’s relations with the papacy, especially with Pope Adrian I, were positive 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 threatened by Muslim and Bulgar pressure 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. And he enjoyed a vague role as protector of the Christian establishment in Jerusalem. By boldly and resourcefully combining the traditional role of warrior king with aggressive diplomacy based on a good grasp of current political realities, Charlemagne elevated the Frankish kingdom to a position of leadership in the European world.