Piper returned to the United States in January 1944 and assumed duty as operations officer on the staff of 5th Marine Division under Major General Keller E. Rockey at Camp Pendleton, California. He participated in intensive training of the division and later assumed duty as divisional logistics officer. Piper went overseas with 5th Marine Division in mid-1944 and participated as colonel in the Iwo Jima campaign in February and March 1945, during which he was responsible for the planning and organization of logistics. For his part in the Iwo Jima Campaign, Piper received the Legion of Merit with Combat "V".