On this Earth, Theodore Roosevelt was reelected President of the United States as the Progressive Party candidate in 1912, only to be assassinated before taking office when personally breaking a labor strike at the Chicago Union Stockyards with the Rough Riders. Following this, his vice president, Charles Foster Kane takes power, and gradually leads the United States into greater levels of oppression, class division and bureaucratic incompetence and corruption - including an earlier entry into the First World War in 1914 and the assassination of his rival candidate, Woodrow Wilson, during the 1916 election campaign.
Gradually, by 1917 the United States is unstable politically and socially, with overwhelming civil unrest stemming from the massive (and seemingly pointless) loss of American lives in the mud of the Western Front and the increasing gap between the wealthy 'robber barons' and the poor workers, and the massive corruption and exploitation this has resulted in. The Socialist Party of America led by Eugene V. Debs gains increasing support, and soon the unrest has led to outright civil war, following which Kane is ousted from the White House and a new socialist order, led by Debs, takes over.
The early idealism of this change is misplaced, however; upon Debs' death power is seized by Al Capone, who proceeds to rule over the USSA with a brutal, repressive fist of iron, establishing a cult of personality around himself, exiling and executing his political rivals and ruling the country more brutally and ruthlessly (and incompetently) than any of the robber barons who were previously deposed. Gradually, following the Second World War, the Cold War between the USSA, the United Kingdom and the Russian Empire (ruled by the Tsars, although vaguely more open and democratic than previously) and the war in Indo-China (a Vietnam War-like affair in which armies from Russia and Great Britain attempted to support a somewhat democratic regime under attack from USSA-backed Communist guerrillas), the USSA begins to stagnate economically and socially, before finally collapsing into separate, bickering nations by 1991, leading to an uncertain future for both America and the rest of the world.