The Katyn Massacre was a series of mass executions of nearly 22,000 Polish military officers and intelligentsia carried out by the Soviet NKVD. The Soviet Union initially blamed the massacre on the Nazis after the bodies were discovered in 1943. For decades, the USSR maintained this lie, and the Western Allies—wary of offending Stalin—did not officially challenge the Soviet version. It wasn't until 1990 that the Soviet Union finally admitted responsibility and released the official documents proving that Stalin himself had signed the execution orders. The massacre remains a deep point of historical trauma in Polish-Russian relations.