Leon Trotsky was a Russian revolutionary, Marxist theorist, and Soviet politician. He was one of the leaders of the October Revolution in 1917, which led to the overthrow of the Tsarist government and the establishment of the Soviet Union. Trotsky was also a key figure in the early years of the Soviet Union, serving as People's Commissar for Foreign Affairs and later as the founder and commander of the Red Army. He was eventually expelled from the Soviet Union in 1929 and assassinated in Mexico in 1940.