There is strong evidence that the central Soviet authorities deliberately refrained from a larger accumulation of partisan forces in western Belarus and let Polish underground military structures grow in these lands during 1941–42 in order to strengthen Moscow's relations with the Polish government-in-exile of Władysław Sikorski. A certain level of military cooperation, imposed by the command headquarters, was noted between Soviet partisans and the Polish Home Army, "Armia Krajowa" (AK). Soviet partisans avoided to some extent attacking people of Polish nationality during the terror campaigns in 1942. After the breakdown of diplomatic relations between the USSR and the Polish government-in-exile in April 1943 resulting from the discovery of the Katyn massacre (which the Katyn Commission of April–May 1943 attributed to the Soviets), the situation changed radically. From this moment on, Moscow treated the AK as a hostile military force.