The Katyn massacre was a series of mass executions carried out by the Soviet Union between April and May 1940 in Poland. Nearly 22,000 Polish military and police officers, border guards, and intelligentsia prisoners of war were executed by the NKVD, at Joseph Stalin's orders. Though the killings also occurred in the Kalinin and Kharkiv NKVD prisons 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.