Complex sentence: The main objective of the operation was to capture Cherbourg or Brest in northern France in order to establish a defensible foothold in mainland Europe, which would provide a staging area for a larger invasion force.
Split into simpler sentences: The main objective of the operation was to capture Cherbourg or Brest in northern France in order to establish a defensible foothold in mainland Europe. This foothold was to provide a staging area for a larger invasion force.