In December 1939, the League of Nations, an intergovernmental organization aimed at promoting peace and cooperation among countries, voted to expel the Soviet Union following its military invasion of Finland. This invasion was a part of the Winter War, which began in November 1939 when the Soviet government, led by Joseph Stalin, sought to expand its territory and secure borders against perceived threats. The Soviet Union's aggression against Finland was viewed as a blatant violation of international law and sovereignty. The situation escalated as Finland, a member of the League, successfully resisted the Soviet advance, surprising many with its resilience despite being significantly outnumbered.

The decision to expel the Soviet Union was a landmark moment for the League of Nations, highlighting the organization's limitations and the difficulties in maintaining collective security. It also marked a turning point in Soviet relations with the Western powers. Many nations were alarmed by Stalin's actions and began to reassess their policies in light of the changing geopolitical landscape. The expulsion highlighted the failure of collective security mechanisms and the inability of the League to effectively address acts of aggression, setting the stage for future international conflict.

The expulsion came amid a broader context of rising tensions across Europe, as countries were grappling with the effects of unresolved issues following World War I and the rise of totalitarian regimes. The League’s decision, while significant, did not stop the Soviet Union's military actions nor prevent further escalation in Europe. Instead, it solidified the division between the Soviet Union and other European nations, foreshadowing the geopolitical alignments that would define the decades to come, particularly as World War II loomed on the horizon.