Complex sentence: He could not resist the chance to become also Count of this rich Principality, close to Normandy and hurried there with his army, where he was defeated by the Count of Flanders, and killed in the Battle of Cassel on 22 February 1071.
Split into simpler sentences: He could not resist the chance to become also Count of this rich Principality, close to Normandy. He hurried there with his army, but nevertheless was defeated by the Count of Flanders, losing his life in the Battle of Cassel on 22 February 1071.