After the attack on Pearl Harbor in 1941, Murphy's older sister helped him to falsify documentation about his birthdate in order to meet the minimum-age requirement for enlisting in the military. Turned down initially for being underweight by the Army, Navy and the Marine Corps, he eventually was able to enlist in the Army. He first saw action in the 1943 Allied invasion of Sicily; then in 1944 he participated in the Battle of Anzio, the liberation of Rome, and the invasion of southern France. Murphy fought at Montélimar and led his men on a successful assault at L'Omet quarry near Cleurie in north-eastern France in October.