Einhard. The Life of the Emperor Charles. Chapter 8.
Charles himself fought but two pitched battles in this war, although it was long protracted ‹ one on Mount Osning, at the place called Detmold, and again on the bank of the river Hase, both in the space of little more than a month. The enemy were so routed and overthrown in these two battles that they never afterwards ventured to take the offensive or to resist the attacks of the King, unless they were protected by a strong position. A great many of the Frank as well as of the Saxon nobility, men occupying the highest posts of honor, perished in this war, which only came to an end after the lapse of thirty-two years. So many and grievous were the wars that were declared against the Franks in the meantime, and skillfully conducted by the King, that one may reasonably question whether his fortitude or his good fortune is to be more admired. The Saxon war began two years before the Italian war; but although it went on without interruption, business elsewhere was not neglected, nor was there any shrinking from other equally arduous contests. The King, who excelled all the princes of his time in wisdom and greatness of soul, did not suffer difficulty to deter him or danger to daunt him from anything that had to be taken up or carried through, for he had trained himself to bear and endure whatever came, without yielding in adversity, or trusting to the deceitful favors of fotune in prosperity.
Text source: The Life of Charlemagne by Einhard. Translated by Samuel Epes Turner.