A Soviet political leader of the twentieth century. Stalin ruled the Soviet Union, often with extreme brutality, from the death of Lenin in the early 1920s until his own death in the early 1950s. His policies of collectivization, which abolished private ownership, were followed by political purges in which thousands of Communist party officials were killed, usually on trumped-up charges of treason. (See Stalin's purge trials.) Stalin led the Soviet Union in its costly victory in World War II; the country again lost huge numbers. President Franklin D. Roosevelt of the United States and Prime Minister Winston Churchill of Britain met with Stalin in 1945 to produce the Yalta agreement. Stalin's expansion of Soviet influence after World War II contributed to the cold war.