Clive Candy goes to Germany to fight a duel over propaganda about the British treatment of people in South Africa in the Boer War. Many of the cited things he was dueling over were in fact true. "Concentration camp" was first used to describe British camps in South Africa in 1899-1902.
The filmmakers wanted Laurence Olivier to play Clive Candy, but he was prevented from being furloughed from the Navy by Prime Minister Winston Churchill, who didn't want the film to be made. Churchill didn't want to bolster the production with an actor and star of Olivier's caliber, as he felt the movie was critical of a type of British patriot. Olivier was allowed to take a leave from the Navy to make a film about William Shakespeare's patriotic King Henry V in Henry V (1944). Roger Livesey was cast instead. A generation later he played Olivier's father, Billy Rice, in The Entertainer (1960), though he was actually less than a year older than Olivier.
According to the directors, the idea for the film did not come from the comic strip by David Low, but from a scene cut from their previous film, One of Our Aircraft Is Missing (1942), in which an elderly member of the crew tells a younger one, "You don't know what it's like to be old."
One of the earliest films to directly refer to The Wizard of Oz (1939) (one of the characters sings part of "We're Off To See the Wizard"), proving rather conclusively that "Oz" was more successful and popular on its first release than is sometimes claimed.
Towards the end of the movie, Candy's assistant Murdoch, played by John Laurie, tells Candy that he has joined the Home Guard. 25 years later Laurie would go on to play Pte. Frazer in all 80 episodes of Dad's Army (1968), a British sitcom revolving around the misadventures of the members of a local group of Home Guard during World War II.
Although uncredited, a very young Patrick Macnee (who would famously go on to play John Steed in The Avengers (1961)) can be briefly seen near the beginning of the film as one of the British soldiers who abduct Clive Candy from the Turkish bath and bundle him into the back of the army truck whilst Candy protests that "The war games haven't started yet". Macnee's distinctive voice also says a couple of words in this scene which helps identify him. He would have been approximately 21 years old when this scene was filmed.