The Formation of the USSR
During WW1, Russia was getting beaten badly by Germany during WW1. The people were very angry and rose up against the tsar. The monarchy was overthrown and replaced by a democracy, which was overthrown and replaced by a communist government. Russia would then erupt into a civil war, between the communist Red Army and the noncommunist White Army in 1917. The USSR would then form in 1922 during the Russian Civil War, when it absorbed several of its surrounding countries. The civil war would soon end later that year.
(below) The Countries that made up the USSR
(above) The Communist Revolution in Russia.