Complex sentence: The war had its roots in a dynastic disagreement dating back to the time of William the Conqueror, who became King of England in 1066 while retaining possession of the Duchy of Normandy in France.
Split into simpler sentences: The war had its roots in a dynastic disagreement. The English Royal family was descended from William the Conqueror, who became King of England in 1066 while retaining possession of the Duchy of Normandy in France.