Complex sentence: Sampson intended to intercept and destroy the Spanish squadron and then move on to attack secondary shore targets-San Juan's castles, forts and batteries and was unaware that Topete had already eluded them and slipped his squadron into the Bay of Santiago.
Split into simpler sentences: Sampson intended to intercept and destroy the Spanish squadron and then move on to attack secondary shore targets-San Juan's castles, forts and batteries. He was unaware that Topete had already eluded them and slipped his squadron into the Bay of Santiago.