Complex sentence: He proposed avoiding a set battle, except under very favorable conditions, until the arrival of reinforcements: by maintaining his fleet in being, he would force the French to remain in the area and prevent them from undertaking other operations.
Split into simpler sentences: He proposed avoiding a set battle, except under very favorable conditions, until the arrival of reinforcements. By maintaining his fleet in being, he would force the French to remain in the area and prevent them from undertaking other operations.