Espionage & Resistance
The Polish Resistance Exposed the Katyn Massacre in 1943 -- and Stalin Killed 22,000 Polish Officers for It
In 1943, the Nazi German forces uncovered mass graves at Katyn Forest near Smolensk, Russia, containing the bodies of approximately 4,400 Polish officers who had been captured by the Soviet Union during the 1939 invasion of Poland. The Polish Underground (Home Army) independently investigated the site and confirmed that it was the Soviets -- not the Germans -- who had committed the massacre. Stalin responded by breaking relations with the Polish government-in-exile and suppressing all information about the massacre. The Soviet Union maintained the official fiction that Germany had committed the killings until 1990.
Source: WW2 Trivia Research