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.
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."
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.
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.
The trivia item below may give away important plot points.
This was one of Powell-Pressburger films that wasn't favored among many Germans especially due to the way the film portrayed World War 1 and National Socialist Germany. This is because it was Germany and her Allies who wanted peace and they sent their peace terms to Great Britain and also America to end World War 1 which were published in several newspapers (October 5, 1916 & December 13-14, 1916). For Example, Chicago Tribune published Germany's peace terms to Great Britain and America on December 13, 1916. But Great Britain's rejection of Germany's peace offers was also published in December 13, 1916 of Chicago Tribune. On December 20, 1916 (Chicago Tribune), Great Britain again announced their rejection of the peace terms of Germany and her Allies with the headline "War Must Go On: Lloyd George to Germany." Many Germans also disliked how the film portrayed Nazi Germany as evil.