On 17 September 1939, the Red Army invaded Poland without declaring war, breaching the Soviet–Polish Non-Aggression Pact, set to last until 1945.
Poland’s international protests were largely ignored, initiating its fourth partition.
The invasion, a consequence of the Molotov–Ribbentrop Pact signed by Nazi Germany and the Soviet Union on 23 August 1939, led to the Katyń massacre, mass deportations of Poles to Siberia, civilian atrocities, and widespread plunder.
Poland was caught between two ruthless war machines.