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The film opens with the figure of "Miss Columbia" (the statue that serves as the studio's trademark) jumping down off her pedestal because she is frightened by a mouse hiding under her dress. The film ends with the written proclamation: "The end. We Hope." Miss Columbia then climbs back onto her pedestal. After the opening credits roll, an offscreen narrator introduces that history of the Duchy of Grand Fenwick and its inhabitants. The scene in which "Helen" and "Mountjoy" drive off with the bomb ends with footage of a nuclear explosion. The narrator then explains that this is not the end of the film, but that the footage was included to "put audiences in the mood." The action then continues as "Tully" runs after the car.
According to publicity material contained in the film's production file at the AMPAS Library, author Leonard Wibberley had the idea for the novel on which the film is based while working as an editorial writer for the Los Angeles Times. Wibberley, fascinated by the peace treaty negotiated between the United States and Japan, wrote a satirical editorial in which he suggested that Japan was awarded so much aid for losing the war that perhaps it would be better to lose than win. Wibberley later expanded his thesis into a serialized novel titled The Day New York Was Invaded, which ran in Saturday Evening Post on 25 December 1954.
When the novel was published in book form, the title was changed to The Mouse That Roared. Walter Shenson, who was then working as the head of publicity for Columbia Pictures in Britain, was given a copy of the book by actor Tyrone Power. Shenson was so impressed by the book that he bought the screen rights in 1956 and resigned from Columbia in 1957 to devote his energy to producing the film, which marked his debut as a producer. Shenson went on to produce a 1963 sequel titled The Mouse on the Moon, directed by Richard Lester and starring Margaret Rutherford and Terry-Thomas (see AFI Catalog of Feature Films, 1961-70).
A premiere of the film was held for a group of diplomats in Geneva, Switzerland on May 23, 1959. According to publicity materials, the film's interiors were shot at the Shepperton Studios in England and location filming was done in the English Channel and in Surrey, England. Although an October 1958 Los Angeles Examiner news item stated that Columbia had hired Jean Seberg to play one of the female leads and was trying to persuade Kathryn Grant to play the other, the character played by Seberg is the only ingnue in the film. The Life magazine review called Peter Sellers, who was relatively unknown in the United States at the time of the film's release, "the funniest actor England has sent to America since Alec Guinness."