Territories of Poland annexed by the Soviet Union

Temporary borders created by advancing German and Soviet troops. The border was soon readjusted following diplomatic agreements.

17 days after the German invasion of Poland in 1939, which marked the beginning of the Second World War, the Soviet Union invaded the eastern regions of Poland (known as the "Kresy"), which Poland re-established during the Polish–Soviet War, and annexed territories totaling 201,015 square kilometres (77,612 sq mi) with a population of 13,299,000. Inhabitants besides ethnic Poles included Czech, Lithuanian, Belarusian, Ukrainian, Jewish, and other minority groups.

Most of these territories remained within the Soviet Union in 1945 as a consequence of European-wide territorial rearrangements configured during the Tehran Conference of 1943. Poland was “compensated” for this territorial loss with the pre-War German eastern territories much of which had been devastated during the war, and had been looted and pillaged by the Red Army. Communist Poland described the territories as the "Recovered Territories". The number of Poles in the Kresy in the year 1939 was around 5.274 million, but after ethnic cleansing in 1939-1945 by the Nazi Germany, Soviet Union and Ukrainian nationalist forces consisted of approximately 1.8 million inhabitants.[1] The post-World War II territory of Poland was significantly smaller than the pre-1939 land areas, shrinking by some 77,000 square kilometres (30,000 sq mi) (roughly equaling that of the territories of Belgium and the Netherlands combined).

Molotov–Ribbentrop Pact

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.