From Eric J. Goldberg's The Saxon Stellinga Reconsidered:
Charlemagne's conquest of Saxony was a momentous turning point that overthrew the distinctive political structures and Pagan culture of the Saxons. Before the conquest, Franco-Saxon relations had been a checkered history of wars, alliances, and Saxon payments of tribute. By the 770s Charlemagne resolved to incorporate Saxony into his growing empire, apparently in order to settle once and for all border disputes with the Saxons. The result was a series of wars, raids, treaties, and rebellions between 772 and 804 through which Saxony south of the Elbe was gradually incorporated into the Frankish empire. In the oft-quoted words of Einhard: "No war ever undertaken by the Frankish people was more prolonged, more full of atrocities, or more demanding in effort." This was a war of conquest and conversion. Charlemagne equated Saxon submission to Frankish rule with the acceptance of Christianity; according to one Franksih author, Charlemagne resolved "to persevere until the Saxons had either been overcome and subjected to the Christian religion or totally exterminate."
Charlemagne's conquest of Saxony actually fell into two distinct phases (772-85 and 792-804) separated by a seven-year armistice (785-92). Between 772 and 785 the war followed a similar annual pattern: almost every year a group of Saxons revolted and attacked a Frankish church, army, or fortress; the Frankish army then invaded Saxony and put down the rebellion without much difficulty; Charlemagne next negotiated with Saxon optimates and primores; and finally Charlemagne exacted oaths of fidelity and hostages from the Saxons and supervised mass baptisms. Between 777 and 785 Widukind, a Saxon Westphalian nobleman, and his socii repeatedly provoked these Saxon rebellions and eluded the clutches of the Franks by seeking refuge in Denmark. In the end Charlemagne bribed Widukind into submission: in 785 Widukind accepted baptism, and the king of the Franks received him "from the font and honored him with magnificent gifts." By 780 Charlemagne began to extend the Frankish church hierarchy into Saxony. After a series of mass executions in 782, Charlemagne abolished the Old Saxon pagus administration under chieftains and implemented the Grafschaftsverfassung (system of countships) common to the rest of the Frankish kingdoms. This new administration placed Saxony under the governance of comites selected from the Saxon nobilissimi. By 785, therefore, Charlemagne had incorporated all of Saxony south of the Elbe into the Frankish kingdom. After a seven-year peace between 785 and 792, the Saxons revolted again, but this time primarily in the regions north of the Elbe. After a series of military expeditions, Charlemagne finally ended the northern war by deporting all Saxons north of the Elbe and in Wihimondia (the northern regions between the mouths of the Aller and Elbe) to Francia.
Charlemagne practiced two main strategies that proved crucial for his success in the wars against the Saxons. First, he secured key strategic locations, such as Eresburg, Paderborn, and Lippspringe, He also confiscated extensive lands along the Hellweg, the main east-west Saxon road between the Rhine and Paderborn, to ensure communication and troop movement in and out of Saxony. Second, as alluded to above, Charlemagne followed a policy of enticing the Saxon edhilingui with bribes and gifts to accept Christianity and Frankish overlordship, as in the case of his chief opponent, Widukind. As Egil (822) wrote in his Vita Sturmi, "The kind ... converted the greater part of that people to the faith of Christ partly through wars, partly through persuasion, and also partly through bribes." Clearly the prospect of appointment to newly created Saxon countships must have convinced many nobilissimi to ally with Charlemagne.