Complex sentence: The battle was, however, quite costly for both sides, the French losing 9,000 men to the Allies' 19,000, the French failed to follow up on their victory, allowing William to escape.
Split into simpler sentences: The battle was, however, quite costly for both sides, the French losing 9,000 men to the Allies' 19,000. The French failed to follow up on their victory, allowing William to escape.