Marshall served in various legal assignments until the beginning of World War II, when he joined 1st Marine Brigade under Major General Holland Smith. He became Smith's protege and followed him as intelligence officer on the staffs of 1st Marine Division, Amphibious Force, Atlantic Fleet and V Amphibious Corps. Marshall participated in most of the Pacific campaigns (Aleutian Islands, Tarawa, Kwajalein, Saipan, Tinian and Guam), reached the rank of colonel and received the Bronze Star Medal with Combat "V" for his service in Pacific.