Leon Trotsky, born as Lev Davidovich Bronstein, was a Russian revolutionary, Soviet politician, and political theorist. He was a key figure in the 1905 Revolution, October Revolution of 1917, Russian Civil War, and the establishment of the Soviet Union, from which he was exiled in 1929 before his assassination in 1940. Trotsky and Vladimir Lenin were widely considered the two most prominent figures in the Soviet state from 1917 until Lenin's death in 1924.

Ideologically a Marxist and a Leninist, Trotsky's ideas inspired a school of Marxism known as Trotskyism. Trotsky joined the Russian Social Democratic Labour Party in 1898, was arrested and exiled to Siberia for his activities. In 1902, he escaped to London, where he met Lenin. Initially siding with the Mensheviks against Lenin's Bolsheviks, he later declared himself non-factional. During the 1905 Revolution, Trotsky was elected chairman of the Saint Petersburg Soviet, was exiled again to Siberia, escaped in 1907, and lived abroad.

After the February Revolution of 1917, Trotsky joined the Bolsheviks, was elected chairman of the Petrograd Soviet, helped lead the October Revolution, and negotiated the Treaty of Brest-Litovsk to withdraw Russia from World War I. He served as People's Commissar for Military Affairs, built the Red Army, and led it to victory in the civil war.

After Lenin's death, Trotsky became a prominent critic of Joseph Stalin, but Stalin politically outmaneuvered him. Trotsky was expelled from the Politburo, the party, and exiled to Alma Ata, then deported in 1929. He lived in Turkey, France, Norway, and settled in Mexico in 1937. In exile, Trotsky wrote against Stalinism, advocating proletarian internationalism. Trotsky's theory of permanent revolution held that the revolution could only survive if spread to more advanced capitalist countries. He founded the Fourth International as an alternative to the Comintern. Trotsky was assassinated in 1940 in Mexico City by a Stalinist agent.

Though written out of official history under Stalin, Trotsky emerged in the Western world as a hero of the anti-Stalinist left, defending a more democratic, internationalist form of socialism against Stalinist totalitarianism. His leadership of the Red Army is highly regarded, and he is credited for his significant contributions to the military, economic, cultural, and political development of the Soviet Union.