While abortive because of Russia's internal strife, these attempts ruined Russia's relations with the Finns and increased support for Finnish self-determination movements.The outbreak of the First World War and the collapse of the Russian Empire during the Russian Revolution and Russian Civil War of 1917–1920 gave Finland a window of opportunity; on 6 December 1917, the Senate of Finland declared the nation's independence.Despite the signing of the treaty, relations between the two countries remained strained. Mannerheim and Väinö Tanner, the leader of the Finnish Social Democratic Party, were targeted for particular scorn.The Finnish government allowed volunteers to cross the border to support the East Karelian Uprising in 1921, and Finnish communists in the Soviet Union continued to prepare for a revanche and staged a cross-border raid into Finland, called the "Pork mutiny", in 1922. 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 Finnish and Swedish militaries engaged in wide-ranging cooperation, but were more focused on the exchange of information and defence planning for the Åland islands than on military exercises, or the stockpiling and deployment of materiel.Nevertheless, the government of Sweden carefully avoided committing itself to Finnish foreign policy.
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.
was a military conflict between the Soviet Union and Finland in 1939–1940.
It began with the Soviet invasion of Finland on 30 November 1939 (three months after the outbreak of World War II), and ended with the Moscow Peace Treaty on 13 March 1940.
For this reason it was also a major factor in the launching of Operation Weserübung, Nazi Germany's invasion of Denmark and Norway.
The poor performance of the Red Army encouraged Hitler to think that an attack on the Soviet Union would be successful and reconfirmed negative Western opinions of Soviet military.
Finland then agreed to cede more territory than originally demanded by the Soviet Union in 1939; the Soviets, having conquered the areas they demanded from Finland but at a cost of heavier losses in troops than anticipated, accepted this offer.