The reason why is because in the Geass universe, Britain won the American revolution but lost to Napoleon so the government relocated to the USA. It's never really explained in the show so I get the confusion.

>>23768671
Literally explained in the backstory. Napoleon invaded the British isles and overthrew the Tudors (Tudors never fell so the Stuarts and Hannoverians never took the throne). The nobility and thousands of other British citizens fled to North America, elevated a family to a pretender throne and they declared themselves the Holy Britannian Empire, going on an alternate Manifest Destiny in the 19th century that combined tons of what would have been Anglo imperial efforts and American industrial prowess into an imperial engine to conquer the whole of the New World. They also took in other ex-nobles from Europe as Napoleon's regimes took root and gave way to a series of republics plagued with corruption.

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