 During Joseph Stalin 's rule , Soviet propaganda painted Finland 's leadership as a " vicious and reactionary Fascist clique " . Marshal C. G. E. Mannerheim and Väinö Tanner , the leader of the Finnish Social Democratic Party , were targeted for particular scorn . With Stalin gaining absolute power through the Great Purge of 1938 , the Soviet Union changed its foreign policy toward Finland in the late 1930s . The Soviet Union began pursuing the reconquest of the provinces of Tsarist Russia lost during the chaos of the October Revolution and the Russian Civil War . The Soviet leadership believed that the old empire had ideal security and territorial possessions , and wanted the newly christened city of Leningrad to enjoy a similar security . 
