Complex sentence: The treaty was meant to bring peace to Normandy by settling the issue of its much-reduced boundaries and the terms of John's vassalage not only for Normandy, but also for Anjou, Maine, and Touraine.
Split into simpler sentences: The treaty was meant to bring peace to Normandy by settling the issue of its much-reduced boundaries. The terms of John's vassalage were not only for Normandy, but also for Anjou, Maine, and Touraine.