Under the terms of the Molotov-Ribbentrop Pact, adjusted by agreement on 28 September 1939, the Soviet Union annexed all Polish territory east of the line of the rivers Pisa, Narew, Western Bug, and San, except for Wilno country with its capital Wilno (Vilnius), which was given to Lithuania, and the Suwalki region, which was annexed by Germany. These territories were largely inhabited by Ukrainians and Belarusians, with large minorities of Poles and Jews. (see also Curzon line for population statistics). The total area, including the area given to Lithuania, was 201,000 square kilometres, with a population of 13.5 million, of which about 5.2-6.5 million were ethnic Poles.
During 1939-1941 at least 1.8 million of the people inhabiting the region were killed or deported by the Soviet regime, from which at least 60% were Poles, and the second largest group were Jews. Recently few Polish historians, basing moslty on queries in Soviet archives, reduced that number somewhat to about half of million people repressed in 1939-1945.
These areas were conquered by the Nazis in 1941 during Operation Barbarossa. The Nazis divided them up as follows:
- Bezirk Bialystok (district of Bialystok), which included the Bialystok, Bielsk Podlaski, Grajewo, Lomza, Sokolka, Volkovysk, and Grodno counties and was "attached to" (not incorporated into) East Prussia;
- Bezirk Litauen und Weissrussland — the Polish parts of White Russia (today western Belarus), including the Vilnius (Vilna) province of Lithuania, which was incorporated into the Reichskommissariat Ostland;
- Bezirk Wolhynien-Podolien — the Polish province of Volhynia, which was incorporated into the Reichskommissariat Ukraine; and
- East Galicia, which was incorporated into the General Government and became its fifth district.
After the end of World War II, all the territories annexed in 1939 were returned to the Soviet Union, with the exception of Bialystok, which was restored to Poland.
See also Polish areas annexed by Nazi Germany, General Government, Kresy.