The mass execution of twenty thousand Polish POWs by the Soviet security police (the NKVD) is one of the most notorious atrocities of World War II. Stalin and the politburo authorized the executions on March 5, 1940, following their receipt of a memorandum from Lavrenti Beria, the head of the NKVD. Beria reported that NKVD prisons held a large number of Polish army, police, and intelligence officers who were unremittingly hostile to the Soviet system, engaged in anti-Soviet agitation within the camps, and eager to escape and to participate in counterrevolutionary activities. Because these prisoners were all "hardened and uncompromising enemies of Soviet authority," Beria recommended they should all be indicted by a special tribunal of the NKVD, and then shot.

According to NKVD records there were 21,857 such executions during March and April of 1940. Most of the victims were Polish officer POWs who had been captured by the Soviets...