The Duchy of Grand Fenwick is a tiny fictional country created by Leonard Wibberley in a series of comedic novels beginning with The Mouse That Roared (1955), which was made into a 1959 film.

The Duchy takes its name from its founder, the English knight Sir Roger Fenwick who, while employed by France, settled there with his followers in 1370.

In the 1959 film version, she is a parody of Queen Victoria who is still wearing mourning for her husband, Prince Louis of Bosnia-Herzegovina, who has been "missing" from a tiger hunt for 27 years.

They are led by Forester Tully Bascomb, appointed High Constable, and Serjeant-at-Arms Will Buckley (who had World War II experience with the British Army).

The longbow is a vital part of the country's history, its borders originally determined by the distance a platoon of bowmen could shoot in each direction.

(In real life at the time, the extent of a country's territorial waters were defined by the three-mile limit, which is traditionally thought to be based on the effective range of a cannon fired from coastal land toward the sea.)

In the film The Mouse on the Moon, Grand Fenwick is shown to have a small military force dressed similar to the Brigade of Guards and armed with rifles.

Grand Fenwick then plans an attack on the United States, certain this will lead to immediate defeat followed by generous American aid.

Grand Fenwick forms an alliance of small nations, the Tiny Twenty, and uses its control of the bomb to obtain world peace.

A 1966 television pilot featuring Sid Caesar in the same roles Peter Sellers performed in the movie (namely the head of state Duchess Gloriana XII, Prime Minister and Count Rupert Mountjoy and the military leader Tully Bascomb) was filmed but never picked up as a series.

[6] In the shared alternate history of Ill Bethisad (1997 and after), Grand Fenwick is an actual country located between France and Helvetia (this world's version of Switzerland).

[8] The Royal Archduke of Grand Fenwick is a scheme used by Barney Stinson to seduce an art expert in the How I Met Your Mother episode "The Ashtray".

[14][15] The duchy's history is also different not only because it's on another planet but because the Otherworld Project is based on plausibility and so this version doesn't have the more outlandish elements given to Grand Fenwick in the books or movies.