alternatehistory.com

The Mouse That Roared is a novel about an imaginary country in Europe called the Duchy of Grand Fenwick that comes to defeat the United States.

The tiny (3 miles by 5 miles) European Duchy of Grand Fenwick, nestled in the Alps between Switzerland and France, proudly retains a pre-industrial economy, dependent almost entirely on making Pinot Grand Fenwick wine.

When an American winery makes a knockoff version, "Pinot Grand Enwick," it quickly puts the tiny 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 (since their standing army is tiny and equipped with bows and arrows), the country confidently expects to rebuild itself through the generous largesse that the United States bestows on all its vanquished enemies (as it did for Nazi Germany through the Marshall Plan at the end of World War II.)

Instead, the Duchy defeats the mighty superpower, purely by accident: landing in New York City, almost completely deserted aboveground because of a city-wide disaster drill, the Duchy's invading "army" wanders to a top secret government lab and unintentionally captures the Q-bomb, a prototype doomsday device that could destroy the world if triggered.
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