Polish Operation of the NKVD

The Polish Operation of the NKVD (Soviet security service) in 1937–1938 was an anti-Polish mass-ethnic cleansing operation of the NKVD carried out in the Soviet Union against Poles (labeled by the Soviets as "agents") during the period of the Great Purge. It was ordered by the Politburo of the Communist Party against so-called "Polish spies" and customarily interpreted by NKVD officials as relating to 'absolutely all Poles'.

Source: Wikipedia — Polish Operation of the NKVD (CC BY-SA 4.0)