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.

[57] 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.

[76] However, this sudden change, contrary to previous statements that Soviet demands were minimalist and thus unalterable, had surprised the Finnish government, and lead them to believe more concession may be forthcoming.

[91][92] In 2013, Russian President Vladimir Putin stated at a meeting with military historians that the Soviets had launched the Winter War to "correct mistakes" made in determining the border with Finland after 1917.

Immediately upon arrival in Helsinki, capital of the country, it will be reorganised and its composition enlarged by the inclusion of representatives of the various parties and groups participating in the people's front of toilers.

[102] Convinced of the invasion's forthcoming success, Andrei Zhdanov, chairman of the highest legislative body in the Soviet Union, commissioned a celebratory piece of music from Dmitri Shostakovich, Suite on Finnish Themes, intended to be performed as the marching bands of the Red Army paraded through Helsinki.

[107] And according to Kotkin, Stalin seemed to be genuinely interested in reaching an agreement during the negotiations: he had personally attended six of the seven meetings with the Finns, and had multiple times reduced his demands.

[109] Additionally, Shaposhnikov argued for a fuller build-up, extensive fire support and logistical preparations, a rational order of battle and the deployment of the army's best units.

Zhdanov's military commander, Kirill Meretskov, reported, "The terrain of coming operations is split by lakes, rivers, swamps, and is almost entirely covered by forests....

[117] Under this divided command structure, the lessons of the Soviet Union's "first real war on a massive scale using tanks, artillery, and aircraft" at Khalkin Gol went unheeded.

[118] As a result, Russian BT tanks were less successful during the Winter War, and it took the Soviet Union three months and over a million men to accomplish what Zhukov had managed at Khalkhin Gol in ten days (albeit in completely different circumstances).

[118][119] Soviet generals were impressed by the success of German Blitzkrieg tactics, but they had been tailored to conditions in Central Europe, with its dense well-mapped network of paved roads.

[123][124] On 28 December, when Stalin asked for volunteers to take over military command, Semyon Timoshenko offered himself on the condition that he be allowed to implement Shaposhnikov's initial plan of a focused attack on the Karelian Isthmus to break the Mannerheim Line; it was accepted.

In prewar calculations, the Finnish Defence Command, which had established its wartime headquarters at Mikkeli,[127] had estimated seven Soviet divisions on the Karelian Isthmus and no more than five along the whole border north of Lake Ladoga.

[142] The array of Finnish defence structures that during the war started to be called the Mannerheim Line was located on the Karelian Isthmus approximately 30 to 75 km (19 to 47 mi) from the Soviet border.

[155] For many of the encircled Soviet troops in a pocket (called a motti in Finnish, originally meaning 1 m3 (35 cu ft) of firewood), staying alive was an ordeal comparable to combat.

The terrain on the Karelian Isthmus did not allow guerrilla tactics, so the Finns were forced to resort to the more conventional Mannerheim Line, with its flanks protected by large bodies of water.

[158] According to the Finns, the real strength of the line was the "stubborn defenders with a lot of sisu" – a Finnish idiom roughly translated as "guts, fighting spirit".

The Finns used effective guerrilla tactics, taking special advantage of their superior skiing skills and snow-white layered clothing and executing surprise ambushes and raids.

The Finnish command believed that the Soviets would not attack there, but the Red Army committed two divisions to the Kainuu area with orders to cross the wilderness, capture the city of Oulu and effectively cut Finland in two.

[180] The Finnish troops captured dozens of tanks, artillery pieces, anti-tank guns, hundreds of trucks, almost 2,000 horses, thousands of rifles, and much-needed ammunition and medical supplies.

[182] The Finnish area of Lapland, bestriding the Arctic Circle, is sparsely developed, with little daylight and persistent snow-cover during winter; the Finns expected nothing more than raiding parties and reconnaissance patrols.

The Germans feared losing the iron ore fields in Northern Sweden and threatened to attack at once if the Swedes granted the Allied forces right of passage.

The heated discussion that ensued failed to yield any clear result and the matter went to a vote, in which the party's opinion prevailed and the decision was taken to bring hostilities to an end.

The Finnish government, realizing that the hoped-for Franco-British military expedition would not arrive in time, as Norway and Sweden had not given the Allies right of passage, had little choice but to accept the Soviet terms.

[219] Finnish President Kyösti Kallio resisted the idea of giving up any territory to the Soviet Union, but eventually agreed to sign the Moscow Peace Treaty.

Nevertheless, Mannerheim eventually agreed to establish a small Russian detachment (Russkaya narodnaya armiya, RNA) of 200 men after being introduced to Boris Bazhanov, a high-ranking ROVS member, in person in January 1940.

[240] The Anglo-French Supreme War Council elected to send notes to Norway and Sweden on 27 December, urging the Norwegians and Swedes to help Finland and offer the Allies their support.

[249] On 31 July 1940, German Chancellor Adolf Hitler gave the order to plan an assault on the Soviet Union and so Germany had to reassess its position regarding Finland.

[252] Finland aimed primarily to reverse its territorial losses from the Moscow Peace Treaty and, depending on the success of the German invasion of the Soviet Union, possibly to expand its borders, especially into East Karelia.