Katyn Massacre

The Katyn massacre was a series of mass executions of nearly 22,000 Polish military officers and intelligentsia prisoners of war carried out by the Soviet Union, specifically the NKVD in April and May 1940. Though the killings also occurred in the Kalinin and Kharkiv prisons and elsewhere, the massacre is named after the Katyn Forest, where some of the mass graves were first discovered by German forces. The order to execute captive members of the Polish officer corps was secretly issued by the Soviet Politburo led by Joseph Stalin. (Wikipedia)