On 1 September , barely a week after the pact had been signed , Germany invaded Poland . The Soviet Union invaded Poland from the east on 17 September and on 28 September signed a secret treaty with Nazi Germany to arrange coordination of fighting against Polish resistance . The Soviets targeted intelligence , entrepreneurs , and officers , committing a string of atrocities that culminated in the Katyn massacre and mass relocation to the Gulag in Siberia
Katyn massacre
Katyn massacre
Axis powers