Winter War
Background: 

Finnish-Soviet relations and politics: Until the early 19th century, Finland was the eastern part of the Kingdom of Sweden. From 21 February 1808 to 17 September 1809, the Russian Empire waged the Finnish War against the Kingdom of Sweden, ostensibly to protect the Russian capital, Saint Petersburg. Eventually Russia conquered and annexed Finland, and converted it into an autonomous buffer state. The resulting Grand Duchy of Finland enjoyed wide autonomy within Russia until the end of the 19th century, when Russia began attempts to assimilate Finland as part of a general policy to strengthen the central government and unify the Empire by Russification. Those attempts were aborted because of Russia's internal strife, but they ruined Russia's relationship with Finland. In addition, support increased in Finland for self-determination movements.
World War I led to the collapse of the Russian Empire during the Russian Revolution of 1917 and the Russian Civil War. On 15 November 1917, the Bolshevik Russian government declared that national minorities possessed the right of self-determination, including the right to secede and form a separate state, which gave Finland a window of opportunity. On 6 December 1917, the Senate of Finland declared the nation's independence. Soviet Russia, later the Soviet Union, recognised the new Finnish government just three weeks after the declaration. Finland achieved full sovereignty in May 1918 after a four-month civil war in which the conservative Whites defeated the socialist Reds with the help of the Imperial German Army, pro-German Jägers, and some Swedish troops, in addition to the expulsion of Bolshevik troops.
Finland joined the League of Nations in 1920 and sought security guarantees, but Finland's primary goal was co-operation with the Scandinavian countries, mainly Sweden, and it focused on the exchange of information and on defence planning (the joint defence of Åland, for example), rather than on military exercises or on the stockpiling and the deployment of materiel. Nevertheless, Sweden carefully avoided committing itself to Finnish foreign policy. Finland's military policy included clandestine defence co-operation with Estonia.
The period after the Finnish Civil War to the early 1930s was a politically unstable time in Finland because of the continued rivalry between the conservatives and the socialists. The Communist Party of Finland was declared illegal in 1931, and the nationalist Lapua Movement organised anticommunist violence, which culminated in a failed coup attempt in 1932. The successor of the Lapua Movement, the Patriotic People's Movement, had a minor presence in national politics and never had more than 14 seats of the 200 in the Finnish Parliament. By the late 1930s, the export-oriented Finnish economy was growing and the nation's extreme political movements had diminished.

After Soviet involvement in the Finnish Civil War in 1918, no formal peace treaty was signed. In 1918 and 1919, Finnish volunteers conducted two unsuccessful military incursions across the Soviet border, the Viena and Aunus expeditions, to annex areas in Karelia that according to the Greater Finland ideology would combine all Baltic Finnic peoples into a single state. In 1920, Finnish communists based in Soviet Russia attempted to assassinate the former Finnish White Guard Commander-in-Chief, Marshal Carl Gustaf Emil Mannerheim. On 14 October 1920, Finland and Soviet Russia signed the Treaty of Tartu, confirming the old border between the autonomous Grand Duchy of Finland and Imperial Russia proper as the new Finnish–Soviet border. Finland also received Petsamo Province, 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 Russia in 1921, and Finnish communists in the Soviet Union continued to prepare for revenge and staged a cross-border raid into Finland, the Pork Mutiny, in 1922. In 1932, the Soviet–Finnish Non-Aggression Pact was signed between both countries, and it was reaffirmed for ten years in 1934. Foreign trade in Finland was booming, but less than 1% of it was with the Soviet Union. In 1934, the Soviet Union also joined the League of Nations.

Justification: Soviet General Secretary Joseph Stalin regarded it a disappointment that the Soviet Union could not halt the Finnish Revolution. He thought that the pro-Finland movement in Karelia posed a direct threat to Leningrad and that the area and defences of Finland could be used to invade the Soviet Union or restrict fleet movements. Soviet propaganda then painted Finland's leadership as a "vicious and reactionary fascist clique". Field Marshal Mannerheim and Väinö Tanner, the leader of the Finnish Social Democratic Party, were targeted for particular scorn. When Stalin gained absolute power through the Great Purge of 1938, the Soviets changed their foreign policy toward Finland and began to pursue the reconquest of the provinces of Tsarist Russia that had been lost during the chaos of the October Revolution of 1917 and the Russian Civil War almost two decades earlier. Soviet leaders believed that the old empire's extended borders provided territorial security and wanted Leningrad, only 32 km (20 mi) from the Finnish border, to enjoy a similar level of security against the rising power of Nazi Germany.

Negotiations: In April 1938, NKVD agent Boris Yartsev contacted Finnish Foreign Minister Rudolf Holsti and Finnish Prime Minister Aimo Cajander, stating that the Soviets did not trust Germany and that war was considered possible between the two countries. The Red Army would not wait passively behind the border but would rather "advance to meet the enemy". Finnish representatives assured Yartsev that Finland was committed to a policy of neutrality and that the country would resist any armed incursion. Yartsev suggested that Finland cede or lease some islands in the Gulf of Finland along the seaward approaches to Leningrad, but Finland refused.
Negotiations continued throughout 1938 without results. The Finnish reception of Soviet entreaties was decidedly cool, as the violent collectivisation and purges in Stalin's Soviet Union resulted in a poor opinion of the country. Most of the Finnish communist elite in the Soviet Union had been executed during the Great Purge, further tarnishing the Soviets' image in Finland. Meanwhile, Finland was attempting to negotiate a military co-operation plan with Sweden and hoping to jointly defend Åland.
The Soviet Union and Nazi Germany signed the Molotov–Ribbentrop Pact in August 1939. It was publicly a non-aggression treaty, but it included a secret protocol in which Central and Eastern European countries were divided into spheres of influence. Finland fell into the Soviet sphere. On 1 September 1939, Germany began its invasion of Poland, and two days later, the United Kingdom and France declared war on Germany. On 17 September, the Soviet invasion of Poland began. After the fall of Poland, Germany and the Soviet Union exchanged occupied Polish lands to establish a new border in accordance with the provisions of the Molotov-Ribbentrop Pact. Estonia, Latvia and Lithuania were soon forced to accept treaties that allowed the Soviets to establish military bases on their soil. Estonia accepted the ultimatum by signing the agreement on 28 September. Latvia and Lithuania followed in October. Unlike the three Baltic countries, Finland started a gradual mobilisation under the guise of "additional refresher training". The Soviets had already started intensive mobilisation near the Finnish border in 1938–39. Assault troops thought to be necessary for the invasion did not begin deployment until October 1939. Operational plans made in September called for the invasion to start in November.
On 5 October 1939, the Soviets invited a Finnish delegation to Moscow for negotiations. Juho Kusti Paasikivi, the Finnish envoy to Sweden, was sent to Moscow to represent the Finnish government Furthermore, the negotiations were attended by Stalin in person, signalling the seriousness of the effort. Paasikivi would later recount his surprise over the friendly atmosphere in which the delegation was received, and mentioned the pleasant manners of Stalin towards them.
The meetings began on 12 October, with Molotov's offer of a mutual assistance pact, which the Finns immediately refused. To the Finns' surprise, Molotov dropped the offer and instead proposed an exchange of territory. The offer stipulated that the Finnish-Soviet border on the Karelian Isthmus be moved westward to a point only 30 km (19 mi) east of Viipuri (Russian: Vyborg) and that Finland destroy all existing fortifications on the Karelian Isthmus. Likewise, the delegation demanded the cession of islands in the Gulf of Finland as well as Rybachy Peninsula (Finnish: Kalastajasaarento). The Finns would also have to lease the Hanko Peninsula for 30 years and to permit the Soviets to establish a military base there. In exchange, the Soviet Union would cede Repola and Porajärvi from Eastern Karelia (2120 square miles), an area twice the size as that of the territory demanded from Finland (1000 square miles).
The Soviet offer divided the Finnish government: Gustaf Mannerheim had argued for an agreement, being pessimistic of the Finnish prospects in a war against the Soviet Union.