Soviet history. They were specifically responsible for maintaining the political regime in the Gulag and conducting mass deportations and forced resettlement. The latter targeted a number of ethnic groups that the Soviet authorities presumed to be hostile to its policies and likely to collaborate with the enemy, including Chechens, Crimean Tatars, and Koreans. World War II War crimes by Soviet armed forces against civilians and prisoners of war in the territories occupied by the USSR between 1939 and 1941 in regions including Western Ukraine, the Baltic states and Bessarabia in Romania, along with war crimes in 1944–1945, have been ongoing issues within these countries. Since the dissolution of the Soviet Union, a more systematic, locally controlled discussion of these events has taken place. As the Red Army withdrew after the German attack of 1941 which is known as Operation Barbarossa, numerous reports of war crimes committed by Soviet armed forces against captured German Wehrmacht and Luftwaffe soldiers from the very beginning of hostilities were documented in thousands of files of the Wehrmacht War Crimes Bureau which was established by Nazi Germany in September 1939 to investigate violations of the Hague and Geneva conventions by Germany's enemies. The targets included both collaborators with Germany and the members of anti-Communist resistance movements such as the Ukrainian Insurgent Army (UPA) in Ukraine, the Forest Brothers in Estonia, Latvia and Lithuania, and the Polish Armia Krajowa. The NKVD also conducted the Katyn massacre, summarily executing over 20,000 Polish military officer prisoners in April and May 1940. The Soviets deployed mustard gas bombs during the Soviet invasion of Xinjiang. Civilians were killed by conventional bombs during the invasion. Estonia In accordance with the Molotov–Ribbentrop Pact Estonia was annexed by the Soviet Union on 6 August 1940 and renamed the Estonian Soviet Socialist Republic. The Estonian standing army was broken up, its officers executed or deported. In 1941, some 34,000 Estonians were drafted into the Red Army, of whom less than 30% survived the war. No more than half of those men were used for military service. The rest were sent to labour battalions where around 12,000 died, mainly in the early months of the war. After it became clear that the German invasion of Estonia would be successful, political prisoners who could not be evacuated were executed by the NKVD, so that they would not be able to make contact with the Nazi government. More