While researching the novel in the early 1970s, author Frederick Forsyth pretended he was actually financing a coup d'etat in Equatorial Guinea. The pretense allowed Forsyth access to a number of underworld figures, including mercenaries and arms dealers. Forsyth has since commented that the arms dealers were the most frightening people he has ever met.
John Irvin had previously filmed amidst real life battles when he worked in a television news crew in Vietnam during the 1960s. Irvin once said: "Unlike most young film directors, I've been in battle. Throughout the sixties I went to various war zones and I met numerous mercenaries in Algeria and South East Asia, so I had my own personal strings to draw on. I would say our depiction of mercenaries is pretty accurate. We talked to a lot of mercenaries and were able to get a lot of information from them about how they would have handled the operation."
The "dogs of war" phrase takes its literary origins from William Shakespeare. It appears in Act 3, Scene 1, line 273 of Shakespeare's Julius Caesar. It reads: "Cry 'Havoc!', and let slip the dogs of war". This Shakespearian phrase was actually used as a tagline for this movie.
This movie's Director of Photography, Jack Cardiff once directed another movie about mercenaries in Africa entitled Dark of the Sun (1968), also based on a novel (by Wilbur Smith). That movie was also known as "The Mercenaries".
The toast given by the French member of the mercenary group is "Vive la mort, vive la guerre, vive le sacre mercenaire." ("Long live death, long live war, long live the cursed mercenary.") This is an adaptation of the original Foreign legion toast, "Vive la mort, vive la guerre, vive la Légion Etrangère", which makes lot more sense!