The sequence in which Japanese troops tried to fool the US Marines by wearing their uniforms was taken directly from the source novel. It includes a passage where the Japanese wore American helmets while attempting to penetrate the Marine positions in order to make them think they were fellow Marines.
The film was filmed mostly in the Philippines (the closing credits state it was also filmed in Japan). The "Japanese" troops in the film were actually Filipino Army soldiers.
In an interview with the British "Films and Filming" magazine in October 1970, director Cornel Wilde discussed his on-set methodology: "I used to find so often in Hollywood that there was nothing more tedious than waiting around. Many directors used a stereotypical system of master shot, medium shot, over-shoulder shots, and then close-ups, with long pauses in between for cameras and lights to be adjusted. I got to my dressing room to paint or write--anything to keep my mind alive. So now my policy is to keep three camera crews working simultaneously, so that actors can move from one set-up to the next without delay. I get the occasional protest, but it isn't easy for anybody to complain that I'm working them too hard, because they can see that I'm working harder than anybody else myself."
Several characters are shown carrying M1 carbines incorrectly fitted with bayonets. When the M1 carbine was introduced in WW2 it was intended to serve as a small, light rifle for rear area and support troops whose duties didn't require them to carry a full-sized battle rifle (usually the longer and heavier Garand). Consequently, they were not equipped with a barrel lug necessary for mounting a bayonet. A version of the M1 carbine capable of mounting a bayonet wasn't introduced until the Korean War.
The US Marine Corps could only provide color stock footage from the Pacific Island campaigns for this movie due to a lack of resources available because of the Vietnam War. This footage had deteriorated badly, and a considerable amount of resources from the film's budget had to used for the footage's restoration to make it match this film's footage.