For nearly two years following the invasion, the two occupiers continued to discuss bilateral plans for dealing with the Polish resistance during Gestapo-NKVD Conferences until Germany's Operation Barbarossa against the Soviet Union, in June 1941.

[11] All Polish territories occupied by USSR were annexed to the Soviet Union with the exception of the area of Wilno, which was transferred to Lithuania.

[12] By this new and final arrangement – often described as a fourth partition of Poland,[2] the Soviet Union secured the lands east of the rivers Pisa, Narew, Bug and San.

[15] The Soviet Union never officially declared war on Poland and ceased to recognise the Polish state at the start of the invasion.

[16] As the Soviet Union had not signed international conventions on rules of war, the Polish prisoners were denied legal status.

[17][18] 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.

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.

[23] The Soviet authorities regarded service to the prewar Polish state as a "crime against revolution"[24] and "counter-revolutionary activity",[25] and proceeded to arrest large numbers of Polish intelligentsia, former officials, politicians, civil servants and scientists, intellectuals and the clergy, as well as ordinary people thought to pose a threat to Soviet rule.

Initially aimed primarily at possible political opponents, by January 1940 the NKVD's campaign was also directed against potential allies, including Polish Communists and Socialists.

Those arrested included Władysław Broniewski, Aleksander Wat, Tadeusz Peiper, Leopold Lewin, Anatol Stern, Teodor Parnicki, Marian Czuchnowski and many others.

[28] The wave of arrests and mock convictions contributed to the forced resettlement of large categories of people ("kulaks", Polish civil servants, forest workers, university professors, "osadniks") to the Gulag labour camps and exile settlements in remote areas of the Soviet Union.

[citation needed] The first major operation took place on February 10, 1940, with more than 220,000 people sent primarily to far north and east Russia, including Siberia and Khabarovsk Krai.

[citation needed] The Soviets lobbied the Western Allies to recognize the pro-Soviet Polish puppet government of Wanda Wasilewska in Moscow.

[37] Deportations, though, continued in June 1944, around 40,000 soldiers and Polish Underground State officials who refused to join the Soviet-controlled Army were relocated to the most remote areas of the USSR.

This action largely nullified the earlier political gains from the land reform as the peasants generally did not want to join the Kolkhoz farms, nor to give away their crops for free to fulfill the state-imposed quotas, which undercut nearly everyone's material needs.

Forty-five new faculty members were assigned to Lwów, transferred from other institutions of Soviet Ukraine, mainly the Kharkiv and Kiev universities.

[11] On December 21, 1939, the Polish currency was withdrawn from circulation without any exchange to the newly introduced rouble; this meant that the entire population of the area lost all of their life savings overnight.

[58] The country remained under de facto military occupation for many years to come, controlled by the Soviet Northern Group of Forces, which were stationed in Poland until 1993.

Some 25,000 Polish underground fighters, including 300 top Home Army officers, were captured by NKVD units and SMERSH operational groups in the fall of 1944.

[60] The events of the late 1940s amounted to a full-scale civil war according to some historians, especially in the eastern and central parts of the country (see: the Cursed soldiers).

According to depositions by Józef Światło and other communist sources, the number of members of the Polish underground, rounded up by order of Lavrentiy Beria of the NKVD and deported to Siberia and various gulags in the Soviet Union reached 50,000 in 1945 alone.

[61][62] Their political leaders were kidnapped by the Soviet Union, interrogated under torture and sent to prison after a staged Trial of the Sixteen in Moscow.

[4] Whether the number of victims could have reached or even exceeded 100,000 is only a matter of guessing,[4] considering the traditional taboos among the women incapable of finding "a voice that would have enabled them to talk openly" about their wartime experiences "while preserving their dignity.