Einhard. The Life of the Emperor Charles. Chapter 7.
At the conclusion of this struggle, the Saxon war, that seems to have been only laid aside for the time, was taken up again. No war ever undertaken by the Frank nation was carried on with such persistence and bitterness, or cost so much labor, because the Saxons, like almost all the tribes of Germany, were a fierce people, given to the worship of devils, and hostile to our religion, and did not consider it dishonorable to transgress and violate all law, human and divine. Then there were peculiar circumstances that tended to cause a breach of peace every day. Except in a few places, where large forests or mountain ridges intervened and made the bounds certain, the line between ourselves and the Saxons passed almost in its whole extent through an open country, so that there was no end to the murders, thefts, and arsons on both sides. In this way the Franks became so embittered that they at last resolved to make reprisals no longer, but to come to open war with the Saxons. Accordingly war was begun against them, and was waged for thirty-three successive years with great fury; more, however, to the disadvantage of the Saxons than of the Franks. It could doubtless have been brought to an end sooner, had it not been for the faithlessness of the Saxons. It is hard to say how often they were conquered, and, huimbly submitting to the King, promised to do what was enjoined upon them, gave without hesitation the required hostages, and received the officers sent them fromt he King. They were sometimes so much weakened and reduced that they promised to renounce the worship of devils, and to adopt Christianity, but they were no less ready to violate these terms than prompt to accept them, so that it is impossible to tell which came easier to them to do; scarcely a year passed from the beginning of the war without such changes on their part. But the King did not suffer his high purpose and steadfastness ‹ firm alike in good and evil fortune ‹ to be wearied by any fickleness on their part, or to be turned from the task that he had undertaken; on the contrary, he never allowed their faithless behavior to go unpunished, but either took the field against them in person, or sent his counts with an army to wreak vengeance and exact righteous satisfaction. At last, after conquering and subduing all who had offered resistance, he took ten thousand of those that lived on the banks of the Elbe, and settled them, with their wives and children, in many different bodies here and there in Gaul and Germany. The war that had lasted so many years was at length ended by their acceding to the terms offered by the King; which were renunciation of their natonal religious customs and the worship of devils, acceptance of the sacraments of the Christian faith and religion, and union with the Franks to form one people.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.