Complex sentence: She headed on out of the harbor to patrol offshore for the next week, then joined in the attempt to relieve Wake Island, but Wake was captured by the Japanese before "Lexington's" force could bring aid.
Split into simpler sentences: She headed on out of the harbor to patrol offshore for the next week, then joined in the attempt to relieve doomed Wake Island. But Wake was captured by the Japanese before "Lexington's" force could bring aid.