Territories of Poland annexed by the Soviet Union

Planned and actual divisions of Europe, according to the Molotov–Ribbentrop Pact, with later adjustments

Early in the morning of August 24, 1939, the Soviet Union and Nazi Germany signed a 10-year non-aggression pact, called the Molotov–Ribbentrop pact. 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] In the North, Finland, Estonia and Latvia were assigned to the Soviet sphere.[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.[2][3] Initially annexed by Poland in a series of wars between 1918 and 1921 (primarily the Polish-Soviet War), these territories had mixed urban national populations with Poles and Ukrainians being the most numerous ethnic groups, with significant minorities of Belarusians and Jews.[4] Much of this rural territory had its own significant local non-Polish majority (Ukrainians in the south and Belarusians in the north).[5]

Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed in September 1939 assigned the majority of Lithuania to the USSR.[6] According to the secret protocol, Lithuania would retrieve its historical capital Vilnius, subjugated during the inter-war period by Poland.

The Polish–Soviet border, as of 1939, had been determined in 1921 at the Treaty of Riga peace talks, which followed the Polish–Soviet War.[7] Under the terms of the Molotov–Ribbentrop Pact, two weeks after the German invasion of western Poland, the Soviet Union invaded the portions of eastern Poland assigned to it by the Pact, followed by co-ordination with German forces in Poland.[8][9] See map.

The "need to protect" the Ukrainian and Belarusian majority populations was used as a pretext for Soviet invasion of Eastern Poland (including Western Ukraine and Belarus) carried out in the wake of Poland's dismemberment under the Nazi invasion with Warsaw being besieged and Poland's government being in the process of evacuation.[10] The total area, including the area given to Lithuania, was 201,015 square kilometres (77,612 sq mi), with a population of 13.299 million, of which 5.274 million were ethnic Poles and 1.109 million were Jews.[11] An additional 138,000 ethnic Poles and 198,000 Jews fled the German occupied zone and became refugees in the Soviet occupied region.[12] The borders were finalized in the September 28 German–Soviet Frontier Treaty, most of whose contents were kept secret.

Soviet authorities immediately started a campaign of sovietization.[13][14] Passportization and residence registration of the population in the newly acquired territories began. Inhabitants of Kresy, on whom Soviet citizenship was imposed in November 1939, had to return documents issued by "former Poland" and obtain new citizenship of the USSR. The NKVD used the passportization system to carefully select people still living in Western Belarus and Western Ukraine. 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.[15]

In March 1940, the authorities also decided about the fate of refugees from western Poland, who from September 1939 were in Kresy. 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.

The Soviets organized staged elections,[16] the result of which was to become a legitimization of Soviet annexation of eastern Poland.[17] Soviet authorities attempted to erase Polish history and culture, withdrew the Polish currency without exchanging ruble,[18] collectivized agriculture,[19] and nationalized and redistributed private and state-owned Polish property.[20] Soviet authorities regarded service for the pre-war Polish state as a "crime against revolution"[21] and "counter-revolutionary activity",[22] and subsequently started arresting large numbers of Polish citizens. During the initial Soviet invasion of Poland, between 230,000 to 450,000 Poles were taken as prisoner, some of whom were executed. NKVD officers conducted lengthy interrogations of the prisoners in camps that were, in effect, a selection process to determine who would be killed.[23] On March 5, 1940, pursuant to a note to Stalin from Lavrenty Beria, the members of the Soviet Politburo (including Stalin) signed an order to execute POWs, labeled "nationalists and counterrevolutionaries", kept at camps and prisons in occupied western Ukraine and Belarus. This became known as the Katyn massacre, in all some 22,000 were executed.[23][24][25][26]

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.[27]

During 1939–1941 1.45 million of the people inhabiting the region were deported by the Soviet regime, of whom 63.1% were Poles, and 7.4% were Jews.[12] Previously it was believed that about one million Polish citizens died at the hands of the Soviets,[28] however recently Polish historians, based mostly on queries in Soviet archives, estimate the number of deaths at about 350,000 people deported in 1939–1945.[29] 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.

The Vilnius Region, annexed by Poland in 1920, was transferred to Lithuania on the basis of Lithuania-Soviet Union agreement. Other northern territories were attached to Belastok Region, Hrodna Region, Navahrudak Region (soon renamed to Baranavichy Region), Pinsk Region and Vileyka (later Maladzyechna) Region in Byelorussian SSR. The territories to the south were transferred to the Ukrainian SSR: Drohobych Oblast, Lviv Oblast, Rivne Oblast, Stanislav (later known as Ivano-Frankivsk) Oblast, Ternopil Oblast and Volyn Oblast.

Sectors of prewar Poland under the Nazi German occupational authority

These areas were conquered by Nazi Germany in 1941 during Operation Barbarossa. The Nazis divided them up as follows:

• Bezirk Białystok (district of Białystok), which included the Białystok, Bielsk Podlaski, Grajewo, Łomża, Sokółka, Vaukavysk, and Hrodna counties and was "attached to" (not incorporated into) East Prussia;
• Generalbezirk Litauen — the Vilna Province was incorporated into Lithuania, itself incorporated into the Reichskommissariat Ostland;
• Generalbezirk Weißruthenien — most of the Polish part of White Ruthenia (the western section of modern-day Belarus) was incorporated into the Reichskommissariat Ostland;
• Generalbezirk Wolhynien und Podolien — the Polish provinces of Volhynia and Polesie, which was incorporated into the Reichskommissariat Ukraine; and
• District Galicia, East Galicia, which was incorporated into the General Government and became its fifth district.

During 1943–1944 ethnic cleansing operations took place in Ukraine (commonly known as the Massacres of Poles in Volhynia) which brought about an estimated 100,000 deaths and an exodus of ethnic Poles from this territory.

The Polish and Jewish language population of the regions in 1939 totaled about 6.7 million. During the war, an estimated 2 million persons perished (including 1.2 million Jews). These numbers are included with Polish war losses. 2 million (including 250,000 Jews) became refugees to Poland or the West, 1.5 million were in the territories returned to Poland in 1945 and 1.2 million remained in the USSR.[30] Contemporary Russian historians also include the war losses of Poles and Jews from this region with Soviet war dead.[31]

Curzon-Namier Line's variants. Tehran, 1943

At the end of World War II, the Soviet Union annexed most of the territory it had invaded in 1939.

From 1944 until 1952 the Ukrainian Insurgent Army (UIA) were engaged in an armed struggle against the communists. As a result of the skirmishes between the UIA and Soviet units, the Soviets deported 600,000 people from these territories and in the process 170,000 of the local population were killed in the fighting. See also Operation Vistula.[37]

In June 1951, the Soviet–Polish border was realigned in two areas.

• Nekrich, Aleksandr Moiseevich; Ulam, Adam Bruno; Freeze, Gregory L. (1997), Pariahs, Partners, Predators: German–Soviet Relations, 1922–1941, Columbia University Press, ISBN 0-231-10676-9
• Piotrowski, Tadeusz (2007), Poland's Holocaust: Ethnic Strife, Collaboration with Occupying Forces and Genocide in the Second Republic, 1918-1947, McFarland, ISBN 978-0-7864-2913-4
• Roberts, Geoffrey (2006), Stalin's Wars: From World War to Cold War, 1939–1953, Yale University Press, ISBN 0-300-11204-1
• Wettig, Gerhard (2008), Stalin and the Cold War in Europe, Rowman & Littlefield, ISBN 978-0-7425-5542-6