On this day, Finland, under increasing pressure from both the United States and the Soviet Union, finally declares war on its former partner, Germany.
After the German invasion of Poland, the USSR, wanting to protect Leningrad more than ever from encroachment by the West—even its dubious Nonaggression Pact partner Germany—began demanding control of various disputed areas from Finland, including part of the Karelian Isthmus (the land bridge that gave access to Leningrad). Finland resisted the Soviet pressure. Soviet Premier Joseph Stalin responded by enacting the "small print" of the Molotov-Ribbentrop Nonaggression Pact the USSR had signed with Germany back in August, which gave the USSR free reign in its "sphere of influence." The Soviets invaded Finland on November 30, 1939. (Stalin claimed that Finnish troops opened fire on Soviet troops.)
The Finns stunned everyone by beating back the initial Soviet offensive. Although their resistance consisted of only small numbers of trained soldiers (on skis and bicycles!) the refusal to submit made headlines around the world. President Roosevelt quickly extended $10 million in credit to Finland, while also noting that the Finns were the only people to pay back their World War I war debt to the United States in full. But by the time the Soviets had a chance to regroup and send in massive reinforcements, the Finnish resistance was spent. In March 1940, negotiations with the Soviets began, and Finland signed the Treaty of Moscow, which handed over control of the Karelian Isthmus.
As tension increased between Germany and the USSR, Finland saw in Hitler a possible ally in gaining back its lost territory. German troops were allowed on Finnish soil as the Germans prepared for their invasion of the Soviet Union—a war that the Finns joined. Although Finnish troops captured large areas of East Karelia back from the Soviet Union, they were reluctant to trespass the old borders of 1939 and help Germany in the siege of Leningrad.
But repeated German setbacks resulted in putting the Soviet Union on the offensive again. Shortly after the Red Army broke through to the Karelian Isthmus in June 1944, the Finnish president, Risto Ryti, resigned. (Around this same time, the United States broke off relations with Finland after repeated demands that Ryti renounce his alliance with Germany were rebuffed.) Ryti's successor, Gustaf Mannerheim, immediately sued for an armistice with the Soviet Union. This was signed on September 19, 1944; Finland agreed to the terms of the 1940 Treaty of Moscow and to throw all German troops off Finnish soil. The final act of capitulation came on March 3, 1945, with a formal declaration of war against the already dying Germany.