Finland gained independence from the dying Russian Empire in 1917 with the recognition of the Bolshevik government, which had taken control of the country that same year. The following decades saw tense relations, marked by the Finnish civil war between conservatives and communists, supported by the Soviets, and territorial disputes in the Karelia region, where a pro-Finnish movement formed on the Russian side. Although they signed a non-aggression agreement in 1932, they soon broke it.
The Ribbentrop-Molotov Pact was signed by the Soviet Union and Germany months before World War II began. It put Finland in the Soviet sphere of influence. After the German invasion of Poland in September 1939, the USSR moved quickly to send its army into neighboring countries. The Social Democratic government of Finland, led by Risto Ryti, refused. The Soviets were worried that Leningrad, which is now called St. Petersburg, was too close to the Finnish border and could be attacked. The USSR chose to take over Finland because of all of these strategic reasons.
November 29, 1939: Winter war begins between the Soviet Union and Finland
The Soviets said that the Finns bombed one of their border posts in the village of Mainila on November 26, 1939. Finland said that the facts were not true, so diplomatic ties between the two countries were broken. Historians have shown that these claims are not true. The USSR was just looking for a reason to start the war.
Tensions rose by the 29th, and the next day, the Red Army invaded Finland and bombed Helsinki and other cities, starting the Winter War. Within a few weeks, the Soviets took over part of neighboring Karelia and set up the Finnish Democratic Republic. The puppet government of the Finnish Democratic Republic was led by Otto Kuusinen, a Finnish communist leader who had escaped Iosif Stalin's purges.
Even though the Soviet army was bigger, Stalin's plans for a quick victory fell through. The Finns, led by General Carl G. E. Mannerheim, had better winter gear, knew the terrain and were good at skiing. This helped them move more quickly through the snow and stay warm in temperatures as low as -40 °C. Thousands of Soviet soldiers were freezing to death at the same time.
This gave the Finns an advantage, and they were able to hold off the Red Army on several fronts, such as Lapland, where the weather was worse. Overall, the Soviets won in February 1940, thanks to their strong military and the fact that the Finnish army had focused most of its efforts on the Karelian isthmus. Since they were tired and didn't have any help from France, the UK, or Germany, the Finns started to talk with Moscow about making peace.
The Cold War in Finland, 1945-47: The End of the Continuation War
In March 1940, Finland and the Soviet Union signed the Moscow Peace Treaty, which gave the Soviet Union the Karelian Isthmus, the northern part of Lake Ladoga, the easternmost islands in the Gulf of Finland, and control of the Hanko peninsula for thirty years. Finland lost more than 10% of its land area, and more than half a million people had to leave their homes because of this. More than 25,000 Finnish soldiers and around 300,000 Soviet soldiers died during the Winter War, though the exact numbers are still disputed.
Peace didn't last long, though, because Finland quickly used World War II to try to get back the land it had lost. In June 1941, it let German troops pass through its territory. Soon after, it started its offensive, which started the Continuation war. This second war ended with an armistice in 1944 and peace in 1947. The borders from 1940 were kept, but Finland had to give up even more land.
Also, the peace meant that it had to break ties with Germany, so it joined the Allies and fought against the Nazis in the Lapland War of 1944. Even so, Finland managed to stay independent from the USSR, which used it as a buffer state between it and Western Europe. Finland stayed in the Western bloc during the Cold War, but it kept close ties with Moscow.