War against the Kingdom of Sweden, ostensibly to protect the Russian capital, Saint Petersburg, and eventually led to the conquest and annexation of Finland and its conversion 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 relations with Finland and increased support for Finnish self-determination movements. World War I led to the collapse of the Russian Empire during the Russian Revolution of 1917 and to 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 until 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