Why did the Soviet Union attack Finland in the Winter War?

The Soviet Union attacked Finland in the Winter War primarily to bolster the security of Leningrad (now St. Petersburg) against the perceived threat of potential German aggression. Following the German-Soviet Nonaggression Pact, which placed Finland within the Soviet sphere of influence, the Soviet government sought to push its border westward on the Karelian Isthmus.123

To achieve this defensive buffer, the Soviet Union demanded that Finland cede several islands in the Gulf of Finland, relinquish a portion of the Karelian Isthmus, and grant a 30-year lease for a naval base at Hanko. Although the Soviets offered to exchange other territories, the Finnish government rejected these demands, leading the Soviet Union to launch its attack on November 30, 1939. This military action was part of a broader Soviet effort to consolidate its sphere of influence in Eastern Europe following the outbreak of World War II.123