on that issue, that the existing Soviet documentation actually makes such cooperation improbable and that it is reasonable to say that Germany did not know about the Katyn massacre until the mass graves were analysed by the Katyn Commission. The fourth and last meeting took place in March 1940 in Kraków. According to some historians, it was part of the Zakopane Conference. This event was described by General Tadeusz Bór-Komorowski, commander of Armia Krajowa in his book Armia Podziemna (The Secret Army). In it, he describes how a special delegation of NKVD came to Krakow, which was going to discuss with Gestapo how to act against the Polish resistance. The talks lasted for several weeks. Russian historian Alexandr Nekrich describes formal military cooperation agreement signed on 20 September 1939 where both sides committed to "cleansing of hostile population" and "liquidation" of Polish resistance. Historian Wojciech Materski points out that there is evidence of clandestine murder operations conducted by both Soviet and German forces in 1939–1940 across occupied Poland, however, there is no evidence of direct connection between the NKVD prisoner massacres and the German AB-Aktion in Poland leading to the massacre of several thousand prominent Poles in the same time-frame. See also Chronicles of Terror Intelligenzaktion Katyn massacre Nazi crimes against the Polish nation Polish areas annexed by Nazi Germany Polish Operation of the NKVD Soviet repressions of Polish citizens (1939–1946) Territories of Poland annexed by the Soviet Union References Further reading Bor-Komorowski, Tadeusz (2011) [1951]. The Secret Army. New York: Macmillan. . 1939 conferences 1939 in international relations 1940 conferences 1940 in international relations Germany–Poland relations Germany–Soviet Union relations Gestapo NKVD Poland in World War II Poland–Soviet Union relations World War II conferences