And from The Rise of Western Christendom by Peter Brown...
Charlemagne proved to be a man of truly "Napoleonic" energy and width of vision. He was constantly on the move and constantly planning. In one year alone (in 785) he covered 2,000 miles, pacing the frontiers of his new dominions. Such energy boded ill for the Old Saxons. The fate of the Pagan Saxons was crucial to Charles' new concept of Christian empire. Not only were Saxons Pagan, they were a surprisingly aggressive warrior confederacy whose raids affected precisely the areas in central Germany werhe Frankish settlement and a Frankish style of life had begun to be established.
As had once been the case along the Roman limes, so now in the eighth century, part of the danger posed by the Saxon challenge came from the fact that Franks and Saxons had drawn closer to each other. Saxon noblemen had already come to adopt a large measure of Frankish customs. Yet, like King Radbod [of Frisia], they clung all the more tenaciously to Paganism so as to differentiate themselves from the Franks. It was all the more essential for the prestige of the Carolingian family that the Saxons, who come to adopt so much of Franksih ways, should be declared to be outside the pale as Pagans, and that, as Pagans, they should be well and truly defeated.
In 772, Charlemagne led the Franks into Saxony. They were said to have desecrated the great intertribal sanctuary of the Irminsul, the giant tree which uphead the world. They rode home again, with much plunder, in time for the hunting season in the Ardennes. Next spring the Franks were in northern Italy. In 774, Charles became king, also, of the Lombards. He even made a short visit to Rome. It was the first time that a Frankish king had set foot in Rome. It was also the first time since the fifth century that a western ruler of such power had been greeted in Rome with the sort of elaborate ceremonies which the Romans know so well how to put on. Charles entered Saint Peter's and, next day, was led through the gigantic basilica churches of the city. In return, Charles proved to be a generous donor. An influx of Frankish silver marked a dramatic recovery in the fortunes of the popes, which was made plain by an unprecedented boom in buildings and repairs.
But it was in Germany, and not in Italy, that Charles showed himself to be a ruler as determined to be obeyed in all matters as any Roman emperor had been. The Saxon war was fought along the same routes into northern Germany as had been taken the legions of Augustus. But this time, unlike Augustus who lost his legions in the Teutoburger Wald, Charlemagne won. It was an unusually vehement war, characterized by the storming, one after another, of well-defended hill-forts. The very flexibility of the kingless society of the Old Saxons prolonged the misery. Total surrender of the Saxons as a whole was impossible. Fifteen treaties were made and broken in 13 years. One Saxon nobelman, Widukind, was able to avoid submission for decades on end. He fled to the Danes and involved even the Pagans of Frisia in his resistance.
For a decade, and entire Frankish order was challenged in the north. Charles found himself forced to take over more territory than he had, perhaps, at first intended to do. He pressed on from the Weser to the Elbe, entering the northern healthlands as far as the Danes. The populations of whole areas were forcibly relocated. In 782, he had 4,500 Saxon prisoners beheaded at Verden, southeast of Bremen....
In 785, Widukind finally submitted and accepted Christian baptism. In the same year, Charles issued his Capitulary on the Region of Saxony. A Capitulary was a set of administrative rulings "from the word of mouth of the king," grouped under capita, short headings. These were very different in their brusque clarity from the long-winded rhetoric of Roman imperial edicts. They registered, in writing, the invisible, purely oral shock wave of the royal will. The royal will was unambiguous. In theory at least, the frontier was now definitively closed. No other rituals but those of the Christian Church could be practiced in a Frankish province."If anyone follows pagan rites and causes the body of a dead man to be consumed by fire ... let him pay with his life.A small body of clergymen (notably Alcuin, a Saxon from Boniface's Britain, who was himself connected with the family of Willibrod) were challenged by the brusqueness to restate, more forcibly than ever before, a view of Christian missions which emphasized preaching and persuasion. But, in fact, when it came to Charlemagne's treatment of the Saxons, most later writers took no notice of Alcuin's reservations. They accepted the fact that, as befitted a strong king, Charlemagne was entitled to preach to the Saxons 'with a tongue of iron' -- as a later Saxon writer put it without a hint of blame. Force was what was needed on a dangerous frontier. Education began, rather, at home. IN the reigns of Charlemagne and his successors, a substantially new Church was allied with a new political system, both of which were committed, to a quite unprecedented degree, to the "correction" and education of their subjects.
"If there is anyone of the Saxon people lurking among them unbaptized, and if he scorns to come to baptism and wishes to absent himself and stay a pagan, let him die."