On November 26, 1939, Mainilla, a Russian frontier village in the Karelian Isthmus, was attacked by artillery fire. The Soviets put the blame for the attack on the Finnish forces positioned just across the border, and then demanded that Finland issue an apology and move back its forces 12-16 miles from the border. When the Finnish government denied any involvement and refused to move back its forces, the Soviet Union repealed the Soviet-Finnish non-aggression pact, and on November 29, 1939, cut diplomatic relations with Finland.
By then, Stalin was impatient and ready to go to war, as large numbers of Soviet forces had already been brought forward in September-October 1939 and were massed along the 600-mile Soviet-Finnish border. With the deployment of first-line assault forces in November 1939, the Red Army was poised to attack. The Soviet invasion force totaled 540,000 troops, 3,000 tanks, and 3,000 planes, an overwhelming superiority in numbers over the Finnish Army by the ratio of 3:1 in manpower, 100:1 in tanks, and 30:1 in planes.
(Taken from Winter War – Wars of the 20th Century – World War II in Europe)
Background On August 23, 1939, Soviet Foreign Minister Vyacheslav Molotov and German Foreign Minister Joachim von Ribbentrop signed the German-Soviet Nonaggression Pact, which included a secret protocol where the two sides agreed to partition between their countries eastern and central Europe into respective spheres of influences. Included in the Soviet sphere were the eastern half of Poland, the Baltic States Estonia and Latvia, and Finland, these territories being formerly part of the Russian Empire but had achieved their independences during the tumultuous final stages of World War I.
At once, Adolf Hitler and Joseph Stalin went to work on the agreement, first against Poland, with the German Wehrmacht invading from the west on September 1, 1939 triggering World War II against the Allies Britain and France, and the Soviet Red Army attacking from the east on September 17, 1939. In early October 1939, the Polish Army was defeated, and independent Poland de facto ceased to exist, with its territories partitioned by Germany and the Soviet Union.
In September-October 1939, the Soviet Union applied strong diplomatic pressure on the Baltic States, and quickly cowered Lithuania, Latvia, and Estonia into signing mutual assistance agreements that allowed Soviet troops and bases in their countries. In June 1940, Soviet forces occupied the Baltic States, and after socialist governments came to power in Soviet-controlled elections held in July 1940, Lithuania, Latvia, and Estonia were incorporated into the Soviet Union in August 1940.
In October 1939, as in the Baltic States, the Soviet Union strong-armed Finland to gain strategic and territorial concessions. Finland had been part of the Russian Empire, annexed by the latter in 1809 as the Grand Duchy of Finland but with broad autonomy. By the mid-19th century, a Finnish nationalist movement had emerged in the capital Helsinki, which grew in opposition to the Empire’s Russification policies, and gradually adopted nationalist sentiments. Then in December 1917, taking advantage of the political turbulence in Russia by the ongoing World War I and the Russian Civil War (which began in November 1917), Finland declared independence, but soon became mired in a bitter civil war between the pro-democratic White Guards and pro-Bolshevist Red Guards. In May 1918, the White Guards prevailed, and Finland achieved full independence.
During the interwar period, Finland sought mutual security guarantees among its Nordic neighbors Sweden and Norway and two Baltic States Estonia and Latvia, but realized that no effective assistance could be expected from them if war broke out. As well, the western powers Britain and France had no strategic interests in the region. Finland then sought collective security in its membership in the League of Nations and its officially declared policy of strict neutrality.
In 1932, Finland also signed a non-aggression pact with the Soviet Union, which was extended to ten years in 1934. Even so, relations between Finland and the Soviet Union remained detached, even guarded, not least because of ideological differences and the lingering suspicion generated by the Finnish Civil War where the Soviets had supported the Red Guards, and Germany the White Guards. Finland distrusted the Soviets, particularly since the latter harbored and supported the exiled Finnish communist movement, while the Soviet Union regarded the ruling right-wing conservative Finnish government as fascist and reactionary.
While officially neutral, Finland appeared to be pro-German, because of German assistance during the Finnish Civil War, which raised Soviet suspicions. Soviet mistrust was furthered by a number of events: in 1937, when a German naval flotilla arrived in Helsinki, in 1938, when Finland held celebrations honoring German support during the civil war, and in 1939, when Franz Halder, the German Army chief of staff, arrived in Helsinki.
Soviet pressure on Finland for territorial concessions had begun in April 1938, the secret negotiations continuing intermittently until the summer of 1939, with no agreement being reached because of strong Finnish opposition. In June 1939, following the visit of high-level German military officials to Finland, Stalin was convinced that not only was a Soviet-German war imminent, but that German forces would use Finland as a springboard to attack the Soviet Union.
But the Molotov-Ribbentrop Pact quelled Stalin’s concerns and seemingly gave assurance that the Germans would not interfere in Finland. Thus, the Soviets increased their pressure on the Finnish government, in October 1939 releasing the following demands: that the Finnish-Soviet border along the Karelian Isthmus be moved west to a point 20 miles east of Viipuri; that Finnish fortifications in the Karelian Isthmus be dismantled; that Finland cede to the Soviet Union the islands in the Gulf of Finland, the Kalastajansaarento (Rybachi) Peninsula in the Barents Sea, and the Salla area; and that Hanko be leased for 30 years to the Soviet Union, where a Russian military base would be built. In return, the Soviets would cede to Finland Repola and Porajarvi from Eastern Karelia, a territory whose size of 3,400 square kilometers was twice as large as those demanded from Finland.
For Stalin, the Soviet-Finnish negotiations must address the security guarantees for Leningrad, since the city was located just 20 miles from the Finnish border and within firing range of Finnish heavy artillery. Stalin wanted to adjust the border here further to the west into Finland, with the ceded territory serving as a buffer zone between the two nations. However, the Finnish government saw these territorial demands as the first step to an eventual Soviet take-over of Finland. On October 6 and 10, the Finnish government issued a call-up of reserves and effectively conducted a general mobilization, fearing that the Soviet demands would be tantamount to Finland meeting the same fate as the Balkan States. The negotiations, though conducted openly, were characterized by great mutual distrust: the Finns believing that the Soviet offer was merely a first step to gobble up Finland, and the Soviets who believed that Finland would side with Germany in a future war.
The Finns presented a counter-offer, agreeing to cede territory in the Karelian Isthmus that would double the distance of the Finnish border to Leningrad. But by then, Stalin was in no mood for more talks and was determined to use armed force, deciding that the Finns were negotiating in bad faith.