The 25,000 soldiers and non-commissioned officers were assigned to forced labor (road construction, heavy metallurgy). Once at the camps, from October 1939 to February 1940, the Poles were subjected to lengthy interrogations and constant political agitation by NKVD officers such as Vasily Zarubin. The Soviets encouraged the Poles to believe they would be released, but the interviews were in effect a selection process to determine who would live and who would die. According to NKVD reports, the prisoners could not be induced to adopt a pro-Soviet attitude. They were declared "hardened and uncompromising enemies of Soviet authority". On March 5, 1940, a note to Joseph Stalin from Beria saw the members of the Soviet Politburo — Stalin, Vyacheslav Molotov, Lazar Kaganovich, Mikhail Kalinin, Kliment Voroshilov, Anastas Mikoyan and Beria — signed an order for the execution of "nationalists and counter-revolutionaries" kept at camps and prisons in western Ukraine and Belarus. This execution became known as the Katyn massacre.where 22,000 perished  Diplomatic relations were, however, re-established in 1941 after the German invasion of the Soviet Union forced Joseph Stalin to look for allies. Thus the military agreement from August 14 and subsequent Sikorski–Mayski Agreement from August 17, 1941, resulted in Stalin agreeing to declare the Molotov–Ribbentrop Pact in relation to Poland null and void, and release tens of thousands of Polish prisoners-of-war held in Soviet camps. Pursuant to an agreement between the Polish government-in-exile and Stalin, the Soviets granted "amnesty" to many Polish citizens, from whom a military force was formed. Stalin also agreed that this military force would be subordinate to the Polish government-in-exile. This force was known as the Anders Army. From 1943 Poles were recruited into the Berling Army. The third group of Polish prisoners were members of Polish resistance organizations ("Armia Krajowa", or 'cursed soldiers') loyal to the Polish government-in-exile and seen by the Soviets as a threat to their establishment of a power base in Poland. Relatively few were sent to the Soviet Union (although there were notable exceptions, see Trial of the Sixteen); most were transferred to the Polish communist security forces and prisons, or enlisted in the Berling Army.