Conducted by the Soviet Union's NKVD under the directives of Joseph Stalin, the operation resulted in the tragic demise of approximately 22,000 Polish officers and intellectuals in the spring of 1940. While the executions took place at various locations, the Katyn Forest remains the most infamous, as it was here that the Nazis uncovered the mass graves in 1943.

RIGHT: Katyn, Mednoye, Russia today: Polish Military Cemetery in memorial complex. Source War History Networl license. Click to enlarge.  Video: Katyn massacre

The Soviet Politburo secretly sanctioned the execution of the Polish officer corps. The victims included roughly 8,000 officers taken prisoner during the Soviet invasion of Poland in 1939, 6,000 police officers, and an additional 8,000 members of the Polish intelligentsia, whom the Soviets deemed as potential adversaries. This group encompassed ethnic Poles, Ukrainians, Belarusians, and between 700-900 Polish Jews. The Polish officer class, reflecting Poland's multicultural society, was deliberately targeted in Stalin's wider plan to quash any possible opposition. 

The slaughter commenced on 3 April 1940, with detainees from the Kozelsk camp being transferred to the Gniezdowo station. Subsequently, they were conveyed in prison ambulances to the forebodingly named Kozia Góra, or Goat Mountain, within the Katyn Forest. In this secluded location, they were systematically shot in the back of the head, and their remains were interred in mass graves. The sheer methodical and impersonal nature of the killings intensified the atrocity, with the more robust prisoners being restrained and blindfolded prior to execution.

At Kozelsk, the roster of the deceased spanned military and societal notables, including generals, professors, doctors, lawyers, engineers, educators, journalists, and even the female aviator, Janina Lewandowska. The executions at Starobielsk and Ostashkov mirrored this procedure, with inmates being taken to Kharkiv and Kalinin (present-day Tver), respectively, for execution in acoustically insulated cells at night. Their corpses were then taken to adjacent woods and buried in previously excavated pits.