The Katyn massacre was a series of mass executions of Poles carried out by the Soviet Union between April and May 1940. Though the killings also occurred in the NKVD prisons in Kalinin, Kharkiv and elsewhere, the massacre is named after the Katyn forest, where some of the mass graves were first discovered by Nazi German forces in 1943. Nearly 22,000 Polish military and police officers, border guards, intelligentsia, and prisoners of war were executed by the NKVD, on Joseph Stalin's orders.