Possibly due to the bad critical reaction, it's been a long held belief by many people that the movie was also a box office flop. Actually, it was one of John Wayne's biggest box office successes, attracting millions of moviegoers and ending up being the eleventh highest grossing movie of 1968.
Late in the movie John Wayne can be seen to wrap his rappelling rope through a carabineer the wrong way. Called a "fatal hookup" in the Army, this would result in an immediate fall once weight was applied.
Scenes were filmed with Vera Miles as John Wayne's wife but they were cut before release by the studio. The scene only took one morning to shoot, but the film was considered overlong, and that scene was judged easiest to cut. Batjac, Wayne's company, offered her $10,000 for her work. When she refused that, they then offered her a new car, which she also declined to accept. Wayne made up for this by casting Miles in his next film Hellfighters (1968).
In 1967 John Wayne wrote to Democratic President Lyndon Johnson requesting military assistance for his pro-war film about Vietnam. Jack Valenti told the President, "Wayne's politics are wrong, but if he makes this film he will be helping us." Wayne got enough firepower to make The Green Berets (1968), which became one of the most controversial movies of all time.
Warner Bros. were concerned about letting John Wayne direct the movie because of the fact that his previous directorial effort, The Alamo (1960), had been an expensive flop. They therefore only agreed to let him do the film if he agreed to co-direct with a more experienced director, and Wayne chose Ray Kellogg who, despite having only ever directed B-movies, the studio accepted due to his track record as a second unit director on a number of major studio releases.
The defensive battle that takes place during the second half of the movie is very loosely based on the Battle of Nam Dong, during which two Viet Cong battalions attacked a small outpost in the Central Highlands of South Vietnam which was defended by a mixed force of Americans, Australians and South Vietnamese troops on July 6, 1964. After the successful defense of the outpost, the commanding officer, CPT Roger Donlon, was awarded the Medal of Honor.
The character Colonel Mike Kirby is based on the real life person Lauri Törni, who later on called himself Larry Thorne. Lauri Törni 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 emigrated to the USA in the late 1940s, and in 1954 joined the US Army, and in November 1963 he joined the 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). Larry Thorne's remains were found in 1999, and formally identified in 2003.
The three leads - John Wayne, David Janssen and Jim Hutton - all died within slightly over eight months of one another: Hutton on June 2, 1979, Wayne on June 11, 1979, and Janssen on February 13, 1980.
All of the enemy Viet Cong and North Vietnamese solders are armed with single-shot rifles and almost no automatic weapons. This was plausible during the early years of the Vietnam war as many Viet Cong were armed with weapons of World War II British or American origin sold to them by the Chinese. The common AK-47 assault rifle used by the Viet Cong and North Vietnamese began appearing in the mid 1960s, but there were very few in Hollywood warehouses to be used a props during that time.
The Pentagon was in the process of trying to prosecute original author Robin Moore for revealing classified information in his book. In an attempt to win the Pentagon over to his side, John Wayne bought Moore out for $35,000 and a 5% share in the profits. He then proceeded to commission a screenplay that had little or no relation to Moore's book.