The Polish Operation of the Soviet NKVD security service in 1937–1938 was a mass operation of the NKVD carried out against purported Polish agents in the Soviet Union during the period of the Great Purge. It was ordered by the Politburo against the so-called “Polish spies” and customarily interpreted by the NKVD officials as relating to “absolutely all Poles”. It resulted in the sentencing of 139,835 people, and summary executions of 111,091 ethnic Poles, as well as those accused of working for Poland. The operation was implemented according to NKVD Order № 00485 signed by Nikolai Yezhov. The majority of the victims were ethnically Polish but not all, according to Timothy Snyder, who gives a conservative estimate of 85,000 confirmed Poles executed simultaneously across the country. The remainder were ‘suspected’ of being Polish, without further inquiry.

https://en.wikipedia.org/wiki/Polish_Operation_of_the_NKVD_%281937%E2%80%9338%29