After invading Poland in 1939, (and after long discussion) (?) Germany decided to annex not only all the lands it was forced to return to Poland in 1919–1922, under the Treaty of Versailles (including the "Polish Corridor", West Prussia, the Province of Posen), but also other territories. The council of the Free City of Danzig voted "democratically" to become a part of Germany again (though Poles and Jews had no right to vote and all non-Nazi political parties were banned).

Two decrees by Adolf Hitler (Oct. 8 and 12, 1939) provided for the division of the annexed areas of Poland into the following administrative units:

• Reichsgau Wartheland (initially Reichsgau Posen), which included the entire Poznań Voivodship, most of the Łódź Voivodship, five counties of the Pomeranian Voivodship, and one county of the Warszawa Voivodship;
• the remaining area of the Pomeranian voivodship, which was incorporated into the Reichsgau Danzig-Westpreussen (initially Reichgau Westpreussen);
• Ciechanów District (Regierungsbezirk Zichenau), consisting of the five northern counties of Warszawa Voivodship (Płock, Płońsk, Sterpe (?) , Ciechanów, Mława), which became a part of East Prussia;
• Katowice District (Regierungsbezirk Kattowitz), or unofficially Ost-Oberschlesien (East Upper Silesia), which included Sosnowiec, Będzin, Chryzanów , and Zawiercie counties, and parts of Olkusz and Żywiec counties.

These territories had area of 94 000 km² and a population of 10,000,000 people.

After German attack on the Soviet Union in June 1941, the district of Białystok, which included the Białystok, Bielsk Podlaski, Grajewo, Łomża, Sokółka, Volkovysk , and Grodno counties, was "attached to" (not incorporated into) East Prussia.

For the policies applied on the annexed areas:

• World War II atrocities in Poland

About 860,000 Poles were immediately deported from the annexed territories to the German-controlled remnant of Poland (General Government), while at the same time the Soviet Union began to evacuate Germans from the Baltic, Galicia and Bessarabia according to the Nazi-Soviet population transfers. 400,000 Germans settled down in the re-annexed lands. Poles living on the German re-annexed territories were deprived of their human rights, and faced serious persecutions. By contrast, after World War II Germans living east of the Oder-Neisse Line were expelled to Germany, but those who were former Polish citizens faced trials (see Pursuit of Nazi collaborators).