Robert E. Green signed up for the U.S. Naval Reserve Midshipman School in June of 1940, the same month he turned 21, and graduated as Ensign, USNR a year later. During the war, he served in the Navy in the Pacific, except for six months during 1943 when he returned stateside to pick up a new ship in New York. In the North Pacific, he served at Attu and Kiska in the Aleutian Islands in 1943. In the South Pacific he took part in the Guadalcanal campaign in 1942 and the Battle of Tarawa in 1943. He earned a Silver Star for his service at Tarawa, and earned four battle stars for his service on his first ship, an APA (attack transport). During the final two years of the war, he was first lieutenant of a floating drydock at Manus Island in the Bismarck Sea, where ships were quickly repaired so that they could return to action.