Complex sentence: On February 16, paratroopers and amphibious units simultaneously assaulted the islet of Corregidor-necessary to overrun, because enemy troops there can block the entrance of Manila Bay.
Split into simpler sentences: On February 16, paratroopers and amphibious units simultaneously assaulted the islet of Corregidor. It was necessary to take this stronghold because troops there can block the entrance of Manila Bay.