Russo-Finnish War, 1939-40

Despite the nonaggression part with Hitler, Stalin remained wary of Germany's military power and sought to secure the Baltic flank of the Soviet Union. In September and October, 1939, the tiny countries of Estonia, Latvia, and Lithuania were pressured by the Soviet Union into signing treaties that allowed Soviet troops to be stationed in their territories. (The Soviet Union annexed those nations in 1940.) On October 7, 1939, the Soviets demanded that Finland give up land near Leningrad (St. Petersburg) on the Karelian Isthmus, surrender islands in the Gulf of Finland, and grant the Soviets use of the Hangö (or Hanko) naval base, in exchange for Soviet territory on Finland's eastern border. Finland agreed to all the demands except the use of the naval base, and negotiations ended on November 30 when the Soviet Union invaded Finland.

In the south, Finland was protected by the Mannerheim Line, a series of fortifications named for Field Marshal Carl Gustav Emil Mannerheim, the Finnish commander, and by swampy, lake-filled terrain. The Soviets attempted to flank the southern defenses by attacks along Finland's eastern frontier. Although outnumbered and poorly equipped, the Finns were well trained in winter fighting, and, aided by an unusually severe winter, they repulsed the Soviet attacks. North of the Arctic Circle, the Soviets took Petsamo but were unable to advance further. In the center they were badly beaten at Salla. Two Soviet divisions were destroyed at Suomussalmi and columns moving around Lake Ladoga were thrown back.

The Soviets reorganized in January, 1940, under the leadership of General Gregori M. Stern. Efforts to attack Finland's eastern border were scaled back in order to concentrate on the Mannerheim Line. After massive artillery barrages, the Soviets were able to penetrate the Mannerheim Line at Summa. Koivisto fell on February 26 and Viipuri (now Vyborg) was isolated on March 11. On March 12 Finland signed a peace treaty and surrendered somewhat more territory than the Soviets had originally demanded.