America remained under British rule, and never had it's revolution. Amazingly enough, it turns out that all the America, Fuck Yeah! shit about freedom and the constitution didn't really mean much, since America (now Britannia) developed in much the same way, becoming a world power to the point where the head of the government was moved there. Only, y'know, it was even better off, since they developed mechs and shit, too.
Code Geass is the ultimate expression of the idea that America's ideal of freedom is completely useless. In a world where they never got that inflated sense of what they deserve, they end up accomplishing much, much more than they ever did in reality.
Also, they still have Pizza Hut.