Finland achieved full sovereignty in May 1918 after a four-month civil war, with the conservative Whites defeating the socialist Reds with the help of the Imperial German Army, the pro-German Jägers and some Swedish troops, and the expulsion of Bolshevik troops. Soviet Russia (later the USSR) recognised the new Finnish Government just three weeks after the declaration. On 6 December 1917, the Senate of Finland declared the nation's independence. 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, giving Finland a window of opportunity. World War I led to the collapse of the Russian Empire during the Russian Revolution of 1917 and the Russian Civil War of 1917–1920. These attempts were aborted because of Russia's internal strife, but they ruined Russia's relations with the Finns and increased support for Finnish self-determination movements. The resulting Grand Duchy of Finland enjoyed wide autonomy within the Empire 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 through russification.
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 conquering and annexing Finland, converting it into an autonomous buffer state. Until the beginning of the 19th century, Finland constituted the eastern part of the Kingdom of Sweden. Neutral countries with military bases occupied by Soviet Union in September and October 1939 11.3.2 Finnish, Russian and other languages.7.2 Soviet offensive on the Karelian Isthmus.7.1 Red Army reforms and offensive preparations.3.2 First battles and Soviet advance to the Mannerheim Line.3.1 Start of the invasion and political operations.1.3 Shelling of Mainila and Soviet intentions.1.1 Soviet–Finnish relations and politics.After 15 months of Interim Peace, in June 1941, Nazi Germany commenced Operation Barbarossa, and the Continuation War between Finland and the USSR began.
The poor performance of the Red Army both encouraged German leader Adolf Hitler to believe that an attack on the Soviet Union would be successful and confirmed negative Western opinions of the Soviet military. Finland retained its sovereignty and enhanced its international reputation. Their gains exceeded their pre-war demands, and the USSR received substantial territories along Lake Ladoga and further north. Soviet losses were heavy, and the country's international reputation suffered. Hostilities ceased in March 1940 with the signing of the Moscow Peace Treaty, in which Finland ceded 9% of its territory to the Soviet Union. After the Soviet military reorganized and adopted different tactics, they renewed their offensive in February and overcame Finnish defences. The battles focused mainly on Taipale in Karelian Isthmus, on Kollaa in Ladoga Karelia and on the Raate Road in Kainuu, but there were also battles in Salla and Petsamo in Lapland. Finland repelled Soviet attacks for more than two months and inflicted substantial losses on the invaders while temperatures ranged as low as –43 ☌ (–45.4 ☏). Most sources conclude that the Soviet Union had intended to conquer all of Finland, and use the establishment of the puppet Finnish Communist government and the Molotov–Ribbentrop Pact's secret protocols as evidence of this, while other sources argue against the idea of the full Soviet conquest. The Soviets made several demands, including that Finland cede substantial border territories in exchange for land elsewhere, claiming security reasons-primarily the protection of Leningrad, 32 km (20 mi) from the Finnish border. The League of Nations deemed the attack illegal and expelled the Soviet Union from the organisation. Despite superior military strength, especially in tanks and aircraft, the Soviet Union suffered severe losses and initially made little headway. It began with a Soviet invasion of Finland on 30 November 1939, three months after the outbreak of World War II, and ended three and a half months later with the Moscow Peace Treaty on 13 March 1940. The Winter War, also known as First Soviet-Finnish War, was a war between the Soviet Union (USSR) and Finland.