 After the Soviet involvement in the Finnish Civil War in 1918 , no formal peace treaty was signed . In 1918 and 1919 , Finnish volunteer forces conducted two unsuccessful military incursions across the Russian border : the Viena and Aunus expeditions . In 1920 , Finnish communists based in Soviet Russia attempted to assassinate the former Finnish White Guards Commander @-@ in @-@ Chief General CGE Mannerheim . On 14 October 1920 , Finland and Soviet Russia signed the Treaty of Tartu , confirming the new Finnish – Soviet border as the old border between the autonomous Grand Duchy of Finland and Imperial Russia proper . In addition , Finland received Petsamo , with its ice @-@ free harbour on the Arctic Ocean . Despite the signing of the treaty , relations between the two countries remained strained . 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 . 
