On April 13, 1943, German radio announced the discovery of a mass grave in Katyn forest near Smolensk, where, it was alleged, Soviet security forces had carried out executions of thousands of Polish officers. The claim was denounced by Stalin as a "monstrous invention by the German-fascist scoundrels" designed to sow discord among the war-time allies. The Polish government-in-exile, then in London, appealed to the International Red Cross to conduct an investigation, which provoked the Soviet government to sever relations with the London Poles. The Soviet government continued to deny any responsibility until, under Gorbachev, it admitted that the executions had been ordered by Stalin and carried out by the NKVD. Official documents turned over to the Polish government in 1992 revealed that 4,443 Polish officers had been executed at Katyn and another 16,000 at other sites.
The massacre of Polish officers occurred in 1940 following the partitioning of Poland between Nazi Germany and the Soviet Union in September 1939. To legitimize its invasion of eastern Poland, the Soviet Union claimed it was liberating Ukrainian and Belorussian toilers from their oppressive Polish rulers. In the process of absorbing these regions into the Ukrainian and Belorussian Soviet Republics, the "liberators" arrested tens of thousands of Polish landlords, officials, intellectuals, and officers and sent them to prison camps. Considering it inconvenient to guard, feed, and otherwise keep alive such a large number of Polish prisoners, Lavrentii Beria, People's Commissar for Internal Affairs, organized the execution of some 20,000.
To many Poles, this horrible crime was compounded by the refusal of the Red Army, camped just outside Warsaw in the summer of 1944, to support the uprising of the Polish Home Army against the city's Nazi occupiers. Only after the Nazis crushed the Home Army, which was loyal to the Polish government in London, did the Red Army advance into Warsaw. The Katyn massacre and Soviet perfidy towards the Home Army undoubtedly made it even more difficult than would otherwise have been the case for the pro-Soviet government installed in Poland after the war to achieve popular legitimacy.