The United States of America was the last "superpower", a country whose political, economic, and military influence shaped the actions of every nation on Earth. Before the war the United States had a population of some 300 million people; the US wartime population was approximately 250 million people, roughly 2/5 of the US population served action in some form or another during the War, and over 10 million soldiers lost their lives in combat with the Ch'rix.
The United States collapsed after the Ch'rix bombing of Washington DC and Cheyene Mountain, crippling military and civilian government organization. Several Generals attempted to establish some form of government during the war, but given the limits of communication put on by the Ch'rix occupation they were little more than regional governments.
The US was revived to some extent after the war with the Provisional American Government established to represent the country in the United Nations General Assembly, prior to global unification. General Alder was a born US Citizen, but symbolically renounced his citizenship during his famous "All or None," speech at the UN to maintain formal impartiality as Commander in Chief.
The United States was formally dissolved by the PAG into several of States and Territories within the United Earth, though several nationalist groups persist in an attempt to oneday rebuild the country, though their support is minimal at best.