Leon Trotsky was a prominent Marxist revolutionary and theorist, best known for his role in the Russian Revolution of 1917. He was a key figure in the establishment of the Soviet Union and served as the leader of the Red Army during the Civil War. Trotsky's ideas on permanent revolution and his opposition to Stalin's policies led to his eventual exile from the USSR. Despite his exile, Trotsky continued to write and advocate for his vision of socialism. He founded the Fourth International in 1938, aiming to unite socialist movements worldwide. His legacy remains influential in leftist politics and revolutionary theory.