Complex sentence: Edward III of England, who was at the time claiming dominion over France as well, defeated the French navy at Sluys in 1340, then went on to make raids throughout Normandy, culminating at the Battle of Crécy in 1346.
Split into simpler sentences: Edward III of England, who was at the time claiming dominion over France as well, defeated the French navy at Sluys in 1340. Then, he went on to make raids throughout Normandy, culminating at the Battle of Crécy in 1346.