Complex sentence: Halsey's plans included the use of battleships and cruisers to bombard military facilities and factories; to prepare for these attacks, US Navy submarines sailed into Japan's inshore waters to search for naval mines.
Split into simpler sentences: Halsey's plans included the use of battleships and cruisers to bombard military facilities and factories. To prepare for these attacks, US Navy submarines sailed into Japan's inshore waters to search for naval mines.