Complex sentence: As for the British, they had learned their lesson well--the Royal Navy could not defend British shipping against American raiders; it could not defend British coasts against American raiders; it could not even defend its own fighting vessels against American raiders.
Split into simpler sentences: As for the British, they had learned their lesson well. The Royal Navy could not defend British shipping against American raiders; it could not defend British coasts against American raiders; it could not even defend its own fighting vessels against American raiders.