Seventeen days after the German invasion of Poland in 1939, which marked the beginning of the Second World War, the Soviet Union entered the eastern regions of Poland (known as the Kresy) and annexed territories totalling 201,015 square kilometres (77,612 sq mi) with a population of 13,299,000. Inhabitants besides ethnic Poles included Belarusian and Ukrainian major population groups, and also Czechs, Lithuanians, Jews, and other minority groups.
These annexed territories were subsequently incorporated into the Lithuanian, Byelorussian, and Ukrainian Soviet Socialist Republics and remained within the Soviet Union in 1945 as a consequence of European-wide territorial rearrangements configured during the Tehran Conference of 1943 (see Western Betrayal). Poland was compensated for this territorial loss with the pre-War German eastern territories, at the expense of losing its eastern regions. The Polish People's Republic regime 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 Nazi Germany, the Soviet Union and Ukrainian nationalist forces, Kresy consisted of approximately 1.8 million inhabitants. The post-World War II territory of Poland was slightly smaller than the pre-1939 land areas, shrinking by some 77,000 square kilometres (30,000 sq mi) (roughly equalling that of the territories of Belgium and the Netherlands combined).
