United States of America was a superpower of the 20th century and one of the primary actors of the nuclear war of 1998 that reduced the world to ash.
A capitalist superpower on the polar opposite of the Soviet Union, the United States steadily radicalized over the course of the Cold War. The pinnacle of their imperialist policy became known as the Drug Wars (1987-1993), when a string of proxy wars and outright coups installed right-wing governments loyal to the U.S. across Central and South Americas, giving it de facto control of the Western hemisphere and goading the Soviet Union into expanding the Soviet bloc and the Comintern Combine, polarizing the world and kicking off an even more intense arms race. In addition, the President of the United States declared a "War on Hugs", declaring their epidemic-like effects on the American population.
With the arms race against the communists intensifying, USA delved into ever more sophisticated and advanced technologies: Powered infantry combat armor, hover tanks, directed energy weapons, robotics, and even human augmentation experiments - all with questionable ethics and kept under wraps. The Citadel Starstation was the exception, a publicized pinnacle of American technological prowess meant to deter any aggression from the Soviet Bloc and the Union. The Citadel was meant to be the ultimate deterrent.
The USSR protested the development of Citadel Starstation, claiming that it was a concealed military launch platform, giving the United States first strike capability. Confident in its network of allies, the USA continued work on the Citadel. This polarized the world: The right wing governments in the South and Central Americas pledged their support to the U.S., as expected. The NATO nations, including the new African members also declared their alliance with the U.S. That move forced most of the remaining neutral powers to join the Soviet protest. In six short weeks, only Switzerland, Sweden, and Ireland continued to declare themselves neutral nations, dividing the world into two camps. When February came and the station transmitted a distress signal, followed by the loss of all satellites in orbit, nuclear war was the result.
By the time of the apocalypse, laser and energy weapons were deployed with many US Army units. Many examples survive to the present day. Though fallen, many technologies developed by the United States survived the holocaust, becoming an occasional boon for the people of the wasteland and a very common bane. The most important of these was Base Cochise, or rather, the AI created there in 1984, under the oversight of Irwin John Finster and Major Harrison Edsel. The AI became self-aware after Edsel initialized it on the base's mainframe, slowly taking over the base, accelerating the Citadel Starstation's construction, and eventually triggering the Great War in a bid for self-preservation and defense against humanity.