Berling did not participate in the Polish defence effort during the Invasion of Poland in 1939 . After the city of Vilnius was occupied by the Soviet Union under the terms of the Molotov – Ribbentrop Pact , Berling , along with many other Polish officers , was arrested by the Soviet secret police ( NKVD ) . He remained in prison until 1940 , first in Starobilsk and later Moscow , but since he agreed to collaborate with the Soviets , he avoided execution in the Katyn massacre
Katyn massacre
Katyn massacre
Zygmunt Berling