Upon the outbreak of World War I, some Finns joined the Russian army, others the German army. Following the overthrow of the czarist regime in Russia (February, 1917), Finland was permitted a representative government. After Russia's Bolshevik (Communist) revolution in November, Finland declared itself independent, December 6, 1917. Russia recognized Finland's independence in January, 1918, and in March it was confirmed by the Treaty of Brest-Litovsk.

Finland's first years of independence were difficult ones. The economy was shaky, there was a severe food shortage, and strikes nearly paralyzed the country. Internal conflicts led in 1918 to civil war. On one side were the “Reds,” aided by Russia; on the other were the “Whites,” headed by Baron Carl Gustaf Emil Mannerheim and aided by Germany. The Reds were defeated and Mannerheim was named regent. On June 17, 1919, Finland became a republic, with a new constitution being adopted the next month.

A peace treaty with Russia in 1920 gave Pechenga (Petsamo) to the Finns, providing them with an ice-free port on the Arctic Ocean. In 1921 the League of Nations settled a dispute with Sweden over the Åland Islands in favor of Finland. There was also a dispute with the Russians over Karelia, which they had retained except for the southern tip (the Karelian Isthmus). A Karelian uprising in 1921 was crushed by the Russians.

Finland gradually achieved political and economic stability. Communist strength in the country grew until the government dissolved the party. However, Communists and Socialists took part in later governments. Agrarian reforms and liberal labor legislation were passed and freedom of religion was established. In 1927 an amnesty act freed about 1,200 persons still in prison for their roles in the 1918 civil war. In 1932 a bloodless revolt by a right-wing party was put down. During this period, the Finns built fortifications, called the Mannerheim Line, across the Karelian Isthmus north of Leningrad (St. Petersburg).