Finnish-Russian War, 1939–40, war between Finland and the Soviet Union. After World War II broke out in Sept., 1939, the USSR, never on cordial terms with Finland, took advantage of its nonaggression pact (Aug., 1939) with Germany to make several far-reaching demands on Finland. These demands included the demilitarization of the Mannerheim Line (the Finnish fortification line across the Karelian Isthmus just N of Leningrad [St. Petersburg]), a 30-year lease on Hanko as a naval base, and the cession of several islands in the Gulf of Finland. In return, Russia offered extensive but valueless districts along the eastern border of Finland. Finland balked; negotiations broke down in November. After alleging (Nov. 26) that Finnish artillery had fired on Russian troops, the USSR, denouncing (Nov. 28) the Russo-Finnish nonaggression pact of 1932 and breaking off (Nov. 28) diplomatic relations, attacked on Nov. 30. The Russians first concentrated their efforts on the eastern border of Finland, but the Finns, superior in winter warfare and ably commanded by Field Marshall Mannerheim, repulsed attacks at Lake Ladoga, Suomussalmi, Salla, and Ivalo. World sympathy was with Finland. Sweden and Norway sent volunteers and supplies, and some supplies came from France and Great Britain. Finally, however, small Finland was no match for the USSR. Air bombardments and well-prepared frontal attacks (Feb., 1940) on the Karelian Isthmus brought Finnish resistance to the verge of collapse. In the peace treaty signed on Mar. 12, Finland ceded part of the Karelian Isthmus, Vyborg (Viipuri), and several border territories to the USSR. In June, 1941, warfare was resumed between Finland and Russia and became part of the general world conflagration (see Finland).

See M. Jakobson, The Diplomacy of the Winter War (1961), and A. L. Paley, The Russo-Finnish War (1973).