Territories of Poland annexed by the Soviet Union

Most notably, the pact contained a secret protocol, revealed only after Germany's defeat in 1945, according to which the states of Northern and Eastern Europe were divided into German and Soviet "spheres of influence".

[2] Poland was to be partitioned in the event of its "political rearrangement"—the areas east of the Narev, Vistula and San Rivers going to the Soviet Union while Germany would occupy the west.

[4] Much of this rural territory had its own significant local non-Polish majority (Ukrainians in the south and Belarusians in the north).

[6] According to the secret protocol, Lithuania would retrieve its historical capital Vilnius, subjugated during the inter-war period by Poland.

During the Interbellum period, the Second Polish Republic had carried out an oppressive programme of Polonization against its Ukrainian, Belarusian, and Jewish minorities.

According to Ukrainian-Canadian historian, Orest Subtelny, these events constituted "collective punishment" meted out on thousands of "mostly innocent peasants" and resulted in the exacerbation of animosity between the Polish state and the Ukrainian minority.

Those who did not receive the citizenship or refused to accept it (claiming that they were Polish citizens or not agreeing to enter Ukrainian or Belarusian nationality) were arrested or deported.

Deportation of this group of about 75–80 thousand people, consisting mainly of Jews (about 84%), finally began on June 29, 1940, and lasted for nearly a month.

NKVD officers conducted lengthy interrogations of the prisoners in camps that were, in effect, a selection process to determine who would be killed.

[25][26][27][28] During Perestroika, former top ministers of Stalin such as Lazar Kaganovich and Vyacheslav Molotov claimed that in Katyn, of the 22,000 Polish officers, roughly 3,000 were killed by the NKVD in 1940, while others were later executed by Nazis.

[31] Andrzej Paczkowski puts the number of Polish deaths at 90–100,000 of the 1.0 million persons deported and 30,000 executed by the Soviets.

[34] He offered a smaller section of land, but Stalin declined, telling him that he would allow the exiled government to participate in the Polish Committee of National Liberation.

[35] The Western Allies were unaware of the existence of the secret clause dividing Poland between Hitler and Stalin already in 1939 along the Curzon Line.

[37] On August 16, 1945 the Communist-dominated Provisional Government of National Unity signed a treaty with the USSR to formally cede these territories.