The Soviets invaded Poland on the 17th September and occupied the Eastern Borderlands with a population of over 13 million.
Sovietization was a policy to liquidate the Polish state.
Rigged elections were held and the Eastern Borderlands was annexed to the USSR.
Approximately 200,000 Polish POWs were captured by the Soviets. 21,857 were executed in the Katyn Massacres. Most were sent to the gulags.
Mass arrests of hundreds of thousands of civil leaders, priests, policeman and judges started on the 18th September until June 1941. The NKVD executed thousands of prisoners.
Between 1 and 2 million Polish citizens were deported to Siberia.
Polish institutions were dismantled. The civil administration and Polish officials were replaced by Communists. Polish property, both private and state, was stolen by these committees and nationalised. Farms were requisitioned and collectivized.
Education, media and all aspects of daily life was controlled by the Soviets. Religion was persecuted. Terror reigned.
Ethnic tensions between Poles and other ethnic groups were exploited by the Soviets.
For food and basic necessities became scarce as supplies were sent East by the Soviets.