Aldo Ray's alcoholism was a continual problem during filming, to the extent that John Wayne had to give some of Ray's dialogue to other characters.
John Wayne's character, Col. Mike Kirby, is based on the real-life Lauri Törni, who later on called himself Larry Thorne. He was a Finnish army captain who fought in the Second World War during the Winter War (1939-40) and Continuation War (1941-44) against the Soviet Union. He served on the Eastern Front as a Waffen-SS captain (under the alias Larry Lane) of the Finnish Volunteer Battalion of the Waffen-SS. He emigrated to the U.S. in the late 1940s and in 1954 joined the U.S. Army. In November 1963 he joined Special Forces unit A-734 in Vietnam and fought in the Mekong Delta. He disappeared during a mission in 1965 and was reported MIA (Missing In Action). His remains were found in 1999, and formally identified in 2003. He is the only former Waffen SS member buried in Arlington Nation Cemetery.
John Wayne said he believed the extremely negative reviews probably helped the film's box-office performance. He further said that he felt critics were attacking the war itself rather than his film.
In 1967, John Wayne wrote a letter to Democratic President Lyndon B. Johnson requesting military assistance for his pro-war film about Vietnam. The Defense Department had previously helped other war films like Sands of Iwo Jima (1949) and The Longest Day (1962). Jack Valenti told the President, "Wayne's politics are wrong, but insofar as Vietnam is concerned, his views are right. If he made the picture he would be saying the things we want said." Wayne got enough help from the Defense Department to make this film, which became one of the most controversial movies of all time.
At the beginning of filming George Takei told John Wayne he was strongly against the Vietnam War. Wayne replied that so was half of the cast and crew, and that he had hired Takei for his acting ability and not for his political views.