After long discussion, Germany decided to re-annex not only all the German lands it was forced to surrender to Poland in 1919–1922, under the Treaty of Versailles (including the "Polish Corridor", West Prussia, the Province of Posen and Upper Silesia), 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 Poznan Voivodship, most of the Lodz 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);
• Ciechanow District (Regierungsbezirk Zichenau), consisting of the five northern counties of Warszawa Voivodship (Plock, Plonsk, Sterpe, Ciechanow, Mlawa), which became a part of East Prussia;
• Katowice District (Regierungsbezirk Kattowitz), or unofficially Ost-Oberschlesien (East Upper Silesia), which included Sosnowiec, Będzin, Chryzanow, and Zawiercie counties, and parts of Olkusz and Zywiec counties.

These areas had a surface of 94 000 km² and a population of 10,000,000 people.

After the German attack on the Soviet Union in June 1941, the district of Bialystok, which included the Bialystok, Bielsk Podlaski, Grajewo, Lomza, Sokolka, 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 expel Germans from the Baltic countries. 360,000 Baltic 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 transferred to Germany, but those who were former Polish citizens faced trials (see Pursuit of Nazi collaborators).