Tanner was instrumental in concluding the 1920 Tartu Treaty and the 1940 Moscow Treaty, both of which eased Finland's relationship with the Soviet Union. He transformed the SDP into a party that offered a Scandinavian brand of socialism. His pragmatic leadership helped heal the wounds of the civil war following World War I, and he led his party to power in 1926 and again in 1937. He served as premier of Finland from December 1926 to December 1927.
When the Winter War with the Soviet Union began in November 1939, as foreign minister Tanner championed his government's resistance to Soviet demands. A cabinet minister throughout World War II, he helped rally the Finnish working class behind the war effort.
Following World War II, Moscow considered Tanner its greatest nemesis. On Soviet insistence, in 1946 a Finnish court sentenced him to five and a half years of imprisonment under the vague charge of "war responsibility." Pardoned in 1949, he made an energetic political comeback and was reelected chairman of his party in 1957. Suspicious of Tanner's attempts to oust Urho Kekkonen from the presidency, in October 1961 Soviet leader Nikita Khrushchev staged the so-called Note Crisis in which he demanded that Finland reaffirm its neutrality. This secured the reelection of the incumbent president after his quick moves to placate the Soviets.
In 1963 Tanner retired from the chairmanship of the SDP. He died in Helsinki on 19 April 1966.
Jussila, Osmo, Seppo Hentila, and Jukka Nevakivi. From Grand Duchy to a Modern State: A Political History of Finland since 1809. London: Hurst, 1999.; Rintala, Marvin. Four Finns: Political Profiles. Berkeley: University of California Press, 1969.