Here is another excerpt, this one from Alessandro Barbero's Charlemagne: Father of a Continent, focussing on the religious dimension of the conflict between the Saxons and the Franks:
Charles [Charlemagne] had not set himself the declared aim of converting the Saxons to Christianity right from the very beginning. Before him, his father and grandfather had fought against them, and on each occasion, after having defeated them, they were satisfied with the payment of tribute. Einhard [c.775-840, Frankish courtier and biographer of Charlemagne] who was writing when the wounds had had time to heal and could have easily attributed Charles's campaigns beyond the Rhine to reassuring predestinations, actually asserts in very pragmatic terms that 'there were too many reasons for disturbing the peace, for example the border between us and them crossed an open plain, except in a few places where great forests or mountain chains more clearly divided the two countries. Thus murder, raids, and arson were continuously committed by one side or the other.' In the chronicler's opinion, this insecurity of the frontier with the barbarians inevitably meant that 'in the end the exasperated Franks could no longer be contented with returning each blow with another and decided to wage full-scale war against them.'
It is clear that religious motivations were inextricably bound up with political ones, as since the time of Charles Martel I [c. 688-741], Frankish swords had sustained missionary work beyond the Rhine. One of the conditions that Pepin [714-768] imposed on the defeated Saxons was the guarantee that the Frankish and Anglo-Saxon clergy working in the area would be free to continue their apostolic tasks without hindrance. It must have appeared obvious to some of these missionaries that Charles's war had a religious justification. 'If you do not accept belief in God,' Saint Lebuin told the Saxons, 'there is a king in the next country who will enter your land, conquer it, and lay waste.' But the Saxons obstinately refused to believe, so in the end that king had to make his move.