By the time the film was finished the Japanese had surrendered, so MGM pushed the release date back to December 1945. With the war over, the film opened to enthusiastic reviews but low turnout at the box office. As John Wayne later said, "People had seen eight million war stories by the time the picture came out, and they were tired of them."
Robert Montgomery was a real-life PT skipper in World War 2. He helped direct some of the PT sequences for the film after John Ford broke his leg three weeks into filming. Montgomery finished the film and was complimented by Ford for his work. Ford claimed he couldn't tell the difference between his footage and Montgomery's, who took no screen credit.
Lindsay Anderson tells the following anecdote in his biography of John Ford: when he interviewed Ford in 1950, the latter admitted he did not like They Were Expendable (1945)... and actually never saw the finished movie! He disliked everything - project, shooting, editing without his supervision, music added without his consent, etc. Anderson was surprised because he thought it was a good movie, so Ford told him he might watch it after all...
A few weeks later, Anderson received the following telegram that he kept as a memento: "Saw 'They Were Expendable'. You were right. Ford."
Robert Montgomery was able to draw on his activity as an PT commander (at Guadalcanal and Normandy), as could James Curtis Havens, one of the second unit directors and the film's explosives expert.
During production, John Ford had put John Wayne down every chance he got, because Wayne had not enlisted to fight in World War II. Ford commanded a naval photographic unit during the war, rising to the rank of captain and thought Wayne a coward for staying behind. After months of Ford heaping insults on Wayne's head, co-star Robert Montgomery finally approached the director and told him that if he was putting Wayne down for Montgomery's benefit (Montgomery had also served as a naval officer in the war), then he needed to stop immediately. This brought the tough-as-nails director to tears and he stopped abusing Wayne.