The Winter War was a military conflict between Finlandball (allied with Nazi Germanyball) and the Soviet Unionball. The USSR wanted to annex Finland
just like poor Estonia. The invasion failed horribly however, as the Soviet Union suffered 3x as many casualties as the Finns. The end result was an exchange of territory where Finland gave up 12% of its land to the USSR, but the Soviets suffered a severely embarrassing moment in their history.
The two empires
Until the 19th century, the area what became present-day Finland was a patchwork of two ethnic groups, the finns and the samis, under the control of the Swedish Empire. In 1809, the Russian empire conquered the area, and converted it into an autonomous buffer state. While the area enjoyed wide autonomy, Russia then began to strengthen the central government, and unify the empire through russification. These attempts ruined Russia's relationship with the Finns, and increased Finnish separatist movements and nationalism.
Collapse & Creation
The outbreak of World War 1 gave the Finns and opportunity for independence. The Russian Empire was destabilizing, and waves of protests and strikes ground the empire to a halt. The Russian Revolution and Civil War soon broke out, and the empire collapsed, leaving the different ethnic groups living in the empire to govern themselves. Finland was finally free from foreign rule.
The USSR began to claim parts of Finland's clay, in order to create a buffer against a German attack, as the border was close to Leningradball (now St. Petersburgball). The Soviets tried to negotiate with Finland, to give up his Karelian Isthmus territory in exchange for other land. When Finland denied the offer, the Soviets decided to invade.
The Soviet's Red Army pushed back against Finland's small reserve army through the first months of the conflict. Though the Soviets outnumbered Finland and had superiority with tanks and planes, he had difficulty in Finland's clay, and winter weather. Finland however, had a well led army, knew the terrain well, and had warm, snow white camoflage suits. Therefore, while he was vastly outnumbered, he was able to hold off against the Soviets for a long time.
After months of fighting, Finland began to get tired. The Soviet forces began to recieve fresh reinforcements, and his military was once again overrun. With no chance of help from the Western Allies, Finland was forced to beg for peace and give some territories to Soviets.
The Treaty of Moscow was signed in 1940. The treaty forced Finland to give up 12% of it's land to the USSR, including its second largest city, Viipuri (Vyborg). The League of Nations saw this invasion as illegal, and expelled the Soviet Union on December 14, 1939. The Gulf of Finland islands, Karelian Isthmus, Ladoga Karelia, Salla, and Rybachy Peninsula, and lease of Hanko were given to the Soviet Union, but their reputation was damaged by how bad they performed, especially weak army leadership, and this was exploited by Nazi Germany in Operation Barbarossa in 1941 where Finland participated in to reclaim lost land in the Continuation War.