The communist leader who seized Ukraine and Belarus to form the Soviet Union was Vladimir Lenin. After World War I, while Germany was departing from the territories occupied, Lenin claimed that Ukraine and Belarus were formerly territories of the Russian Empire and thus, he annexed them to the USSR.
He was followed by Joseph Stalin, who was named General Secretary of the Communist Party of the Soviet Union. After Lenin’s sudden death, Stalin took over the control of the Soviet Union on April 3, 1922, and he established a totalitarian communist government until October 16, 1952.