This plan helps Marsilion with his objective
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Marsila is the king of Saragossa, and he's vowed vengeance against the franks because Charlemagne has taken over such a large territory. He ostensibly sendd envoys to speak of false peace while he is in all actuality preparing to assault.
Ganelon involves himself when asked to bring Charlemagne's terms to Marsila, who then plots with him to attack the rear guard. The object is to hurt Charlemagne defensively, but for Ganelon it is more importantly a way to rid himself of his stepson.
Song of Roland