Political map of Russia & the former Soviet Union 6 March 1940 (Winter War): After expanding into Poland (Soviet invasion of Poland) and the Baltic states (Background of the occupation and annexation of the Baltic states), the Soviet Union launched an invasion of Finland (Winter War) in late 1939. Despite their huge advantage in numbers and equipment, the war started disastrously for the Soviets. However they persevered and eventually forced the Finns to come to terms (Moscow Peace Treaty).