On the 17th of September 1939, 78 years ago, the Red Army invaded Poland. It was 16 days after the Nazi Germany invaded Poland from the west: See Battle of Westerplatte: https://www.communications-unlimited.nl/the-battle-of-westerplatte/
The invasion ended on 6 October 1939 with the two-way division and annexation of the entire Second Polish Republic by Germany and the Soviet Union, as a result of the Ribbentrop-Molotov pact signed on 23 August 1939.
The Red Army captured around 230,000 Polish prisoners of war.The Soviets pursued mass persecution in the newly acquired areas. This included the NKVD ( People’s Commissariat for Internal Affairs, the Soviet secret police) staged elections which were conducted in the atmosphere of fear and terror. The Soviets used the results of the elections to legitimize its annexation of eastern Poland. The Soviet Union sent hundreds of thousands of people from this region to Siberia and other remote parts of the Soviet Union in four major waves of deportation between 1939 and 1941. They murdered Polish soldiers and Polish citizens. The Katyn massacre was a series of mass executions of Polish nationals carried out by the the NKVD in April and May 1940. It is estimated that around 22,000 Polish nationals were murdered. The victims were executed in the Katyn Forest in Russia, the Kalinin and Kharkiv prisons. Of the total killed, about 8,000 were officers taken prisoner during the 1939 Soviet Invasion, another 6,000 were police officers, and the rest were arrested Polish intelligentsia.
Source: history.com, Wikipedia