The Soviet Union never officially declared war on Poland and ceased to recognise the Polish state at the start of the invasion. The Soviets did not classify Polish military personnel as prisoners of war, but as rebels against the new Soviet government in today's Western Ukraine and West Belarus. The NKVD and other Soviet agencies asserted their control in 1939 as an inherent part of the Sovietization of Kresy. Approximately 250,000 Polish prisoners of war were captured by the USSR during and after the invasion of Poland. As the Soviet Union had not signed international conventions on rules of war, the Polish prisoners were denied legal status. The Soviet forces murdered almost all captured officers, and sent numerous ordinary soldiers to the Soviet Gulag. In one notorious atrocity ordered by Stalin, the Soviet secret police systematically shot and killed 22,000 Poles in a remote area during the Katyn massacre. Among some 14,471 victims were top Polish Army officers, including political leaders, government officials, and intellectuals. Some 4,254 dead bodies were uncovered in mass graves in Katyn Forest by the Nazis in 1943, who invited an international group of neutral representatives and doctors to examine the corpses and confirm the Soviet guilt. 22,000 Polish military personnel and civilians were killed in the Katyn massacre, but thousands of others were victims of NKVD massacres of prisoners in mid-1941, before the German advance across the Soviet occupation zone.