The Federated Commonwealths of America was born from the English colonies established in North America throughout the 17th and early 18th centuries. As the English monarchy grew more absolutist, a steady flow of immigrants settled in the colonies, despite the threat posed by sims.
As English rule grew more tyrannical, the colonies united and broke away from the mother country, becoming independent in 1738. The independent government they chartered was faithfully modeled on the Roman Republic, the only fully developed model for a functioning republic that was readily available to the Conscript Fathers at the time. The executive officers of the Federated Commonwealth were the Censors. Philadelphia was the national capital.
By 1988, the FCA was a large, populous nation dominating the North American continent, and boasted a high standard of living and level of technology.
The full geography of the FCA is never given, but due to the nature of the series' point of divergence, and the lack of references to any border nations, it is reasonable to assume that the FCA covers all of North America.