On September 17, 1939, when the Polish army was fighting Nazi German invasion in the west, Soviet forces invaded eastern Poland.
“Today, on the anniversary of that unexpected and unlawful aggression, which not only broke Polish resistance but also led to the death of the elite of the officers of the Polish Armed Forces, we pay respects to all those who were captured and forgotten,” said Polish Defence Minister Mariusz Błaszczak on the occasion.
Poland was first invaded by Nazi Germany on September 1, 1939. This invasion was expected and most of the Polish army was deployed to the western borderlands, in order to counter the expected attack. However, this left Poland’s border with the Soviet Union only lightly defended.
By September 17, the German Army had already cut deep into Poland, capturing about a third of the country and assaulting the capital city of Warsaw. Soviet invasion in the East completely broke any chances of effective resistance.
On the night of the invasion, the communist government of the Soviet Union presented a diplomatic note to the Polish government, claiming that the Polish state has fallen and it has no functioning government. This was used as a justification for the invasion.
However, a month before the invasion, on August 23, 1939, the foreign ministers of Nazi Germany and the Soviet Union signed the Ribbentrop-Molotov Pact, which contained a secret protocol, dividing Eastern Europe between themselves. The eastern part of Poland which the Soviets invaded on September 17 fell within the areas assigned to the USSR in the pact.
Following the Soviet invasion of Poland, but before the German invasion of the Soviet Union, the Communist authorities murdered or deported tens of thousands of Poles living in those areas.
About 21,000 of the Polish officers captured by the Red Army were murdered by the Soviet authorities, most notably in Katyń, in present-day Russia. The communist government long denied the responsibility for the crime, only admitting to it in 1990.