Groucho Marx offered the following explanation for the movie's title: "Take two turkeys, one goose, four cabbages, but no duck, and mix them together. After one taste, you'll duck soup the rest of your life."
An early draft of the script introduces Freedonia's new dictator Rufus T. Firefly as an agent for an ammunition company, which led to lots of ammunition-salesman jokes and intertwined with the movie's war theme.
The famous phrase "Well, who you gonna believe, me or your own eyes?" is often referred to as a Groucho Marx quote, but it's really delivered by Chicolini (Chico Marx) while impersonating Rufus T. Firefly (Groucho).
According to The Marx Brothers biographer Joe Adamson, the elaborate "All God's Chillun Got Guns" musical number was mostly improvised on the set, as there is no reference to it in the movie's final script. It's likely Groucho was referring to the old Negro spiritual "I got Shoes", which repeats the line "All of God's children got..." filling "a song", "a robe", "a harp", etc. Groucho just added "Guns".
The movie's line "You know, you haven't stopped talking since I came here? You must have been vaccinated with a phonograph needle." was voted as the #43 of "The 100 Greatest Movie Lines" by Premiere in 2007.
Shortly before this film premiered, the city of Fredonia, New York, complained about the use of its name with an additional "e". The Marx Brothers' response was, "Change the name of your town, it's hurting our picture."
During the battle scene, Groucho Marx wears a total of five different uniforms: a Union soldier's uniform, a Confederate general's uniform, a Boy Scout troop leader's uniform, a Revolutionary War-era British general's uniform, and a Davy Crockett-style outfit.
Although this is usually considered the finest film The Marx Brothers ever made, it was not a hit when first released. In fact, the film performed so poorly at the box office that Paramount, then in serious financial straits, terminated its contract with The Marx Brothers, who then promptly signed with MGM where they produced two of their classic films, A Night at the Opera and A Day at the Races under the aegis of the legendary Irving Thalberg.
Harpo Marx's character was originally called Skippy, but this was changed to Pinky after his role in Horse Feathers, making him one of only two of The Marx Brothers to reprise a role in their films (not counting when they used their own names).