The extras in the film were real American GIs, in the process of being transferred from the war in Europe to the Pacific. Many of them were killed in the fighting on Okinawa--the same battle in which Ernie Pyle was killed by a Japanese machine gunner--never having seen the movie in which they appeared.
This is the film that propelled Robert Mitchum to stardom. It also brought him his only Academy Award nomination.
During WW2, infantry soldier Frank Feldman wrote a letter to his wife, describing how his unit watched the movie one evening away from the battlefield. He wrote that the film was very accurate, and that many soldiers left the viewing "with tears in their eyes." The letter survives to this present day.
War correspondent Ernie Pyle acted as advisor to the film. He was killed not long after the film was completed.
The creator of the G.I. Joe Action figure, Hasbro executive Donald Levin, got the idea for the name from this movie. He was originally going to have several names like Rocky the Marine, Ace the fighter pilot, Salty the sailor. Levin was told to keep it to one and after struggling to name the doll, he saw this movie and then licensed the name.