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The Holy Empire of Britannia is an imperial monarchy and the most prominent superpower within the world of Code Geass, controlling over one-third of the world at the start of the series, which expands as the series progresses. Its homeland is based in North America, and its imperial city is Pendragon. Britannian society is elitist and closely resembles Social Darwinism. Society is arranged by ranks of nobility.

During season 1, the Britannian Empire controls the entire Western Hemisphere (both American continents), New Zealand, recently conquered Japan, and a portion of the Indochina Peninsula in the novels (the maps in later episodes contradict this). Early in season 1 Britannia conquers "Area 18", a desert region that is a part of the Middle East (possibly Egypt in North Africa), but it has not been fully pacified yet. During season 2, Britannia successfully conquers about half of the rival "Euro Universe" superpower; taking over France, Spain, the western half of Africa, and Russia.

Ironically, Britannia does not control the British Isles, as Napoleon managed to successfully conquer the region. Territories conquered by the Britannian Empire are renamed with an "Area" number based on when they were conquered and the native people are referred to by their area number or just called "Numbers"; Japan, being the eleventh territory conquered, is Area 11 and its people are "Elevens", for example. The Numbers are denied many of the rights of Britannian citizens, and are usually left to live in poverty. They can apply for Honorary Britannian citizenship to earn basic rights, but cannot attain the same status as regular citizens.