Charles the Great--Charlemagne--became sole king of Carolingian lands with the death of his brother Carloman in 771. The next 40 years saw forceful foreign engagements to expand borders and consolidate his influence in central and Western Europe, as well as in Rome. He engaged militarily in Italy, Saxony, Spain, Bavaria, against the Avars, and against remaining Byzantine outposts in Italy and the Adriatic. He began with Italy and Saxony, forcing a Papal appeal for help from Rome in 772. Pope Hadrian I (772-795) was again under pressure from encroaching Lombards under King Desiderius. At the same time, the latter had hosted Carloman's widow and children, and may have been intriguing against Charlemagne with Frankish nobles. Thus, Charles invaded Italy in 773 with a huge army, besieging Desiderius in his capital at Pavia until Easter 774. The Lombard was required to surrender himself along with his family and royal treasure. Charles took the title King of The Lombards, with his son Louis the Pious as nominal administrator. By 775, the Duchy of Spoleto recognized his suzerainty, while Benevento held out until 787.
Around the same time, Charlemagne began campaigns in Saxony to the East of Austrasia and Frisia that lasted to the 790s. They were particularly hard. Traditional enemies of the Franks, the Saxons were pagans worshipping the Irminsul sacred tree trunk. They had almost no political unity, and were therefore difficult to defeat as an entire people. For the New King of the Lombards, the goal was territorial conquest, and Christianization. Campaigning began in 772, marching as far as Eresburg on the Lippe and Weser. Franks destroyed Irminsul, and received local chiefs' submission. As soon as Frankish armies left for Italy, Saxons repudiated them and raided on Charles' frontier as far as the monastery of Fritzlar by 774. Charlemegne led a second large expedition in 775 to recoup losses. He defeated a Saxon army at the Weser, and moved to the Oker River. Eastern Saxons (Eastphalians) then submitted, as did southern Saxons. The latter retook the offensive when Charles was in Italy in 776, attacking Frankish Eresburg. By 777, Charles got a group of Saxons to submit to him at his Paderborn settlement and receive Christianity, but there was no permanent subjugation. In 779, the Saxons rose again, raiding as far west as the Rhine near Cologne, destroying Frankish outposts on the Lippe. In 780, Charles swept through the region as far as the Elbe, securing the baptism of numbers of Westphalians and more easterly Saxons. In 782, the Saxons annihilated a Frankish force, leading to yearly Frankish campaigns until 785, when a major victory over the Saxons as well as a massacre of thousands of their warriors brought their King Widukind to surrender, accepting baptism. Smaller uprisings in the 790s no longer undercut Charles' position in the area.
A major aspect of Charles' campaigns had been Christianization, both for its own sake, as well as to make Saxons more docile, loyal subjects of a consciously Christian king. He thus used the church and its structures to support his conquest. Forcing Saxons to accept the new faith, he set up bishoprics and dioceses, endowed new monasteries, and equated pagan relapses with revolt. The Pope and his Irish-English monastics responded by vigorously missionizing the region. Another method of Saxon pacification involved population exchanges. Moving large numbers of them into western Frankish lands, he brought in Frankish peasant colonizers. In addition to numerous garrisons and border troops, Charles granted much land to Frankish counts in the area. Gradually, the region was well integrated.
Even while fighting in Saxony, Charles had been attracted southwest into Spain by the Arab Muslim lords of Barcelona and Zaragoza. The latter feared the powerful Ummayad Amir of Cordoba's expansionary inclinations, and appealed to the most powerful European ruler they could find. Charlemagne saw it as a religious mission to retake lands from infidels. Both Barcelona and Zaragoza reneged, however, and Charles was left facing large Ummayad force on his own. He withdrew, but in crossing the Pyrenees, his rearguard was annihilated ironically by Christian Basques. That was the end of incursions into Spain's interior. In the next years, however, he led repeated campaigns into the area just south of the Pyrenees, giving conquered lands to warrior leaders. Called the Spanish March, it was a strong buffer against the Muslims, and provided a jump-off for Reconquista precursors. In 785, Franksih forces took Gerona, while Louis the Pious, Charles' son and nominal king of Aquitane, took Barcelona in 801.
Shortly after incorporation of Saxony, Charlemagne turned to Bavaria, north of his Italian domains. Though its count had accepted Carolingian authority since Pepin III, its leader had not shown sufficient loyalty, and the region was invaded in 787. Subdued, the region was divided into counties granted to Frankish warriors. This brought The Carolingian Empire up against the Avars who had caused much Byzantine misery. Avar raids into Bavaria and Northeast Italy had begun in 787-88. Though repelled by local forces, such raids continued to 791, when Charles decided to launch a major reprisal against "the excessive and intolerable outrage...against the Holy Church and Christian people." An army under Charles' son Pepin defeated the Avars, at which point a civil war erupted, eliminating them as a threat. Frankish armies marched into Avar territory north of the Danube in 795-796, plundering unopposed.
Pope Hadrian died at the end of 795, and Charlemagne was soon dragged back into Italian politics, this time Papal. Since rescue from Lombard dominance, political power of the Papacy in Rome's environs, and its increasing wealth, made the pontificate a sought-after position, able to bestow benefits on relatives and supporters. Leo III was elected to the Papacy in 796, but he was opposed by relatives of his predecessor. Seizing on rumors (or the reality) of Leo's corruption, his opponents staged a coup in 799, imprisoning him in a monastery. The Pope escaped, however, arriving at Charles' encampment in Paderborn. Charlemagne sent him back to the Holy See with a Frankish escort strong to reseat him, by which time his opponents had accused him before Charlemagne of adultery and perjury, asking that the pope not be reinstated. As the dilemma of who was fit to judge the Pope could not be resolved, the situation went no where until Charlemagne himself came to Rome before Christmas 800, and convoked a synod of Church and civil leaders. Leo took an oath (in the mold of compurgation) affirming his innocence, which the synod accepted. Two days later at Christmas mass, the Pope surprised Charlemagne by crowning him as Emperor of the Romans.