The Soviets further demanded parts of eastern Finland , but the Finnish government refused . The Soviets invaded Finland in November 1939 , yet despite numerical inferiority , the Finns kept the Red Army at bay . International opinion backed Finland , with the Soviets being expelled from the League of Nations . Embarrassed by their inability to defeat the Finns , the Soviets signed an interim peace treaty , in which they received territorial concessions from Finland . In June 1940 , the Red Army occupied the Baltic states , which were forcibly merged into the Soviet Union in August ; they also invaded and annexed Bessarabia and northern Bukovina , parts of Romania . The Soviets sought to forestall dissent in these new East European territories with mass repressions . One of the most noted instances was the Katyn massacre of April and May 1940 , in which around 22,000 members of the Polish armed forces , police , and intelligentsia were executed
Katyn massacre
Katyn massacre
Joseph Stalin