The Soviet Union was formed in 1922 following a bloody civil war that was itself sparked by Russia's defeat in the First Great War of 1914-1919. Following the overthrow of Czar Nicholas II and the institution of a provisional government, Bolshevik forces attempted to seize control of the government. Although opposed by anti-communist "White Russian" forces for many years and plagued by internal strife, the Reds won out and established the USSR after half a decade of brutal fighting.
Initially a poor agrarian state, the Soviet Union rapidly industrialized under the stewardship of Joseph Stalin and became a world leader in military and industry by 1950, when it launched an invasion of Eastern Europe that sparked the Second Great War. While that bloody conflict ended in a stalemate that persists to this day, it did establish the USSR as a major world power and a force to be reckoned with on the international stage.
The Soviet Union is also notable for being the second country to successfully detonate an atom bomb (in 1958, three years after Great Britain) and the first to land a man on the moon (Gherman Titov in 1972).