Complex sentence: On discovering Villaret's departure, Howe realised that the main French fleet was due to sail directly across the patrol lines pursued by Admiral Montagu and that should Montagu meet Villaret, his squadron would be destroyed.
Split into simpler sentences: On discovering Villaret's departure, Howe realised that the main French fleet was due to sail directly across Admiral Montagu's planned route. Howe also knew that should Montagu meet Villaret, his squadron would be destroyed.