The tiny European Duchy of Grand Fenwick, supposedly located in the Alps between Switzerland and France, proudly retains a pre-industrial economy, dependent almost entirely on making Pinot Grand Fenwick wine. However, a California winery makes a knockoff version, "Pinot Grand Enwick", putting the country on the verge of bankruptcy. The prime minister decides that their only course of action is to declare war on the United States. Expecting a quick and total defeat, the country confidently expects to rebuild itself through the largesse that the United States bestows on all its vanquished enemies. With the counterfeit wine as a casus belli, they send a formal written declaration of war, but this is misplaced by the State Department. Receiving no response, the Duchy is forced to muster some troops and hire a ship to stage an actual invasion. Instead, the Duchy defeats the mighty superpower, purely by accident. Landing in New York City, almost completely deserted above ground because of a citywide disaster drill, the Duchy's invading "army" wanders to a top secret government lab and unintentionally captures the "Quadium Bomb" and its maker, Dr. Kokintz, an absent-minded professor who is working through the drill. This "Q-Bomb" has a theoretical explosive potential greater than all the nuclear weapons of the United States and the Soviet Union combined. The invaders from Fenwick are sighted by a civil defence squad and are immediately taken to be "men from Mars" when their mail armor is mistaken for reptilian skin. The American Secretary of Defense pieces together what has happened and is both ashamed and astonished that the United States was unaware that it had been at war for two months. With the most powerful bomb in the world now in the smallest country in the world, other countries are quick to react, with the Soviet Union and the United Kingdom offering their support. With the world at the tiny country's mercy, Duchess Gloriana, the leader of Grand Fenwick, lists her terms: all the nuclear weapons of the powerful nations must go through an inspection by impartial scientists. Continued inspection of the continuing nuclear programs of the world powers will be supervised by Dr. Kokintz, who recalls his identity as a Fenwick-American and accepts repatriation to his ancestral home. Kokintz takes on his new role as scientific director of the "Tiny Twenty", a new superpower of 20 of the world's smallest nations headed by Grand Fenwick. The United States and the other world powers accept these humiliating terms, leading to hope for world peace. As a celebration of the triumphant outcome of the war, Duchess Gloriana and Tully Bascomb are united in marriage. As a sequel to the marriage, Dr. Kokintz accidentally drops the Q-Bomb onto the stone floor of the Grand Fenwick castle dungeon. As a result of this mishap, the scientific director inadvertently discovers that the Q-bomb is, and always has been, a powerless dud. The book concludes with Kokintz deciding to keep this key fact to himself.
Background
Wibberley got the idea from the US peace treaty negotiated with Japan by John Foster Dulles, which included generous amounts of aid to Japan. He wrote an article for the Times which suggested that his native Ireland make a token invasion of the US to get aid. He then developed this into a novel changing Ireland to the Duchy of Grand Fenwick.
Reception
praised the novel as "utterly delightful...a very nearly perfect book, on no account to be missed."
The Mouse That Roared was adapted for the stage by Christopher Sergel in 1963. The play portrays Duchess Gloriana XII as twenty-two years old, as in Wibberley's novel. In this version, Dr. Kokintz is a physics professor at Columbia University and the arrival of Tully Bascomb's invasion force coincides with a campus student protest. Thus, the Fenwick soldiers are mistaken for being eccentric protesters rather than as foreign invaders.
Television pilot
In 1964, Jack Arnold produced a television pilot based on the film, with Sid Caesar playing the three roles that Sellers had played, but it was not picked up for production.