Following the defeat of Poland, tens of thousands of Polish soldiers were interned in camps, with many subjected to forced labor, harsh conditions, and political repression.
While some prisoners were later released or escaped to join resistance movements, others suffered severe mistreatment or were executed, most notably during the Katyn massacre.
[2]: 294 Tens of thousands of Polish POWs were "exchanged" between Germany and the USSR based on their domicile location within the relevant occupation zone (it was one of the topics discussed during the Gestapo–NKVD conferences).
[8]: 35 [2]: 10 Within several months, almost all non-officer prisoners of war (estimates range at 300,000–480,000) were stripped of their POW status and forced to work in Nazi Germany.
[2]: 38–40 Most of them were "civilianized" – released from POW camps, then immediately forced to sign labor contracts; this made them technically ineligible for protection from the Geneva Convention.
After that, the number of Polish POWs in German was reduced to 80,000 officers, NCOs, and other groups deemed unfit or unwanted in the labor force (intelligentsia, minorities including Jews, and several other categories).
[1] Similar to what happened in the Western front against the Germans, many of the Polish POWs held by the Soviets were victims of atrocities; some during the campaign itself.
[23]: 21 Like Germans, Soviets were not prepared to deal with large number of Polish POWs, and conditions in transit camps and during transportation were very poor.
The conditions in the camps were poor; most religious and cultural activities were forbidden, and inmates were subject to interrogations and extensive Soviet propaganda.
[2]: 44 Eventually almost all of them perished in the infamous[2]: 27 Katyn massacre (which claimed the life of over 20,000 Polish citizens; victims also included police personnel and other civilians).
[2]: 45–46 [26][27][28][29] The Soviets claimed that Polish officers were released or escaped; although the massacre site was eventually uncovered by the Germans and used by the Nazi propaganda.
[2]: 47–48 The Katyn massacre was subsequently covered up in communist regimes, including Poland, until the fall of communism, and was only discussed by the Western and Polish émigré historians until that time.
[2]: 27 The reasons for Katyn massacre are still not fully understood by the historians, but there is a general consensus that it was done to deprive a potential future Polish military of a large portion of its talent, and to reduce future opposition to the Soviet rule, and was part of a wider plan of oppression of Polish populace in the occupied territories.