Following the Invasion of Poland at the beginning of World War II, nearly a quarter of the entire territory of the Second Polish Republic was annexed by Nazi Germany and placed directly under the German civil administration. The rest of Nazi-occupied Poland was renamed as the General Government district.Polish Ministry of Foreign Affairs, "German Occupation of Poland" (Washington, D.C.: Dale Street Books, 2014), pp. 12–16. The annexation was part of the "fourth partition of Poland" by Nazi Germany and the Soviet Union, outlined months before the invasion, in the Molotov–Ribbentrop Pact.Maly Rocznik Statystyczny (wrzesien 1939 – czerwiec 1941), Ministerstwo Informacji i Documentacji, London 1941, p.5, as cited in Piotr Eberhardt, Political Migrations in Poland, 1939–1948, Warsaw 2006, p.4

Some smaller territories were incorporated directly into the existing Gaue East Prussia and Silesia, while the bulk of the land was used to create new Reichsgaue Danzig-West Prussia and Wartheland. Of those, Reichsgau Wartheland was the largest and the only one comprising solely the annexed territory.

The official term used by the Nazi authorities for these areas was the "incorporated Eastern territories" (German: Eingegliederte Ostgebiete).The Avalon Project: Documents in Law, History, and Diplomacy. Nazi Conspiracy and Aggression Volume 1 Chapter XIII – Germanization and Spoliation. Yale Law School, Lillian Goldman Law Library. Avalon Project : Nazi Conspiracy and Aggression – Volume 1 Chapter XIII – Germanization and Spoliation They planned for a complete Germanization of the annexed territories, considering them part of their lebensraum."Poles: Victims of the Nazi Era " The local Jewish population was forced to live in ghettos, and was gradually deported to concentration and extermination camps, the most infamous of which, Auschwitz, was located in annexed East Upper Silesia. The local Polish population was to be gradually enslaved, exterminated and eventually replaced by German settlers. The Polish elite especially became subject to mass murder, and an estimated 780,000 Poles were subject to expulsion, either to the General Government or to the Altreich for forced labour. The remaining Polish population was strictly segregated from the German population and subject to a variety of repressive measures. These included forced labour and their exclusion from all political and many cultural aspects of society. At the same time, the local German minority was granted several privileges, and their number was steadily raised by the settlement of ethnic Germans, including those displaced by the Nazi-Soviet population transfers.Polish Ministry of Foreign Affairs, "German Occupation of Poland" (Washington, D.C.: Dale Street Books, 2014), pp. 16–45.

After the Vistula–Oder Offensive in early 1945, the Soviet Union took control over the territories. The ethnic German population either fled the Red Army or were later expelled and the territories became part of the People's Republic of Poland.

Background Already in the fall of 1933 Adolf Hitler revealed to his closest associates his intentions to annex western Poland into an envisioned Greater Germany. In October 1939, a month after the invasion of Poland, Nazi Germany annexed an area of (23.7% of pre-war Poland) with a population of about 10,000,000 people (30% of the pre-war Polish population).Nowa Encyklopedia Powszechna PWN Państwowe Wydawnictwo Naukowe, Warsaw 2004, p. 149, volume 6 The remainder of the Polish territory was either annexed by the Soviet Union (201,000 km2 or 51.6% of pre-war Poland as per the Molotov–Ribbentrop Pact) or made into the German-controlled General Government occupation zone (95,500 km2 or 24.5% of pre-war Poland). A tiny portion of pre-war Poland (700 km2) was annexed by Slovakia.

Since 1935, Nazi Germany had been divided into provinces (Gaue) which had replaced the former German states and Prussian provinces. Of the territories annexed, some were attached to the already existing Gaue East Prussia and Silesia (later Upper Silesia), while from others new Reichsgaue Danzig-West Prussia and Wartheland were constituted. Wartheland was the only Gau constituted solely from annexed territory, Danzig-West Prussia comprised also former German areas and the former Free City of Danzig. The occupied General Government remained outside Nazi Germany.

The annexation violated international law (in particular, the Hague Convention IV 1907).Hague IV SECTION III MILITARY AUTHORITY OVER THE TERRITORY OF THE HOSTILE STATE (Art. 42. and later) Nazi Germany's officials discussed the convention and tried to circumvent it by declaring the war against Poland over prior to the annexation, which in their view made the convention non-applicable.

Administration

On 8 and 13 September 1939, the German military district in the area of Posen, commanded by general , and West Prussia, commanded by general Walter Heitz, were established in conquered Greater Poland and Pomerelia, respectively. Based on laws of 21 May 1935 and 1 June 1938, the Wehrmacht delegated civil administrative powers to Chiefs of Civil Administration (CdZ). Hitler appointed Arthur Greiser to become the CdZ of the Posen military district, and Danzig's Gauleiter Albert Forster to become the CdZ of the West Prussian military district. On 3 October 1939, the military districts centered on and named "Lodz" and "Krakau" were set up under command of major generals Gerd von Rundstedt and Wilhelm List, and Hitler appointed Hans Frank and Arthur Seyß-Inquart as civil heads, respectively. Thus the entirety of occupied Poland was divided into four military districts (West Prussia, Posen, Lodz, and Krakau). Frank was at the same time appointed "supreme chief administrator" for all occupied territories.

Hitler's annexation decree, October 1939

After Hitler's peace offer was rejected by French prime minister Édouard Daladier on 7 October (and rejected by British prime minister Neville Chamberlain on 12 October) a decree was issued by Hitler on 8 October 1939,Dieter Gosewinkel (2011), Einbürgern und Ausschließen. RGBI I, S. 2042: "Erlaß des Führers und Reichskanzlers über die Gliederung und Verwaltung der Ostgebiete" (8 October 1939). provided for the annexation of western Polish areas and the Free City of Danzig. A separate by-law stipulated the inclusion of the area around Suwalki (the Suwalki triangle).Piotr Eberhardt, Political Migrations in Poland, 1939–1948, Warsaw 2006, p.4

The first two paragraphs of the decree established "Reichsgau Posen" in Greater Poland with the government regions (Regierungsbezirk) Hohensalza, Posen, and Kalisch, as well as "Reichsgau West Prussia" () in Pomerelia with the government regions Bromberg, Danzig, and Marienwerder. These government regions were named after the German language names of their chief cities: Hohensalza (Inowrocław), Posen (Poznań), Kalisch (Kalisz), Bromberg (Bydgoszcz), Danzig (Gdańsk), and Marienwerder (Kwidzyn). The annexed territories were twice as large as former Prussian conquests in the Partitions of Poland, also contained twice as many people. Compared to 1914, the border of Reich was extended eastwards by some 150–200 km on average.Czesław Madajczyk. Polityka III Rzeszy w okupowanej Polsce, pp. 19–73, volume 1, Państwowe Wydawnictwo Naukowe, Warsaw, 1970. Despite this fact, Germany used old Prussian propaganda of creating a "German living wall" in Polish territories. On 29 January 1940, Reichsgau Posen was renamed "Reichsgau Wartheland" (Warthegau). Reichsgau West Prussia was renamed "Reichsgau Danzig-West Prussia".

The remaining annexed areas were not made separate provinces but included in the existing provinces of East Prussia and Upper Silesia per § 4 of Hitler's decree. Arthur Greiser was made Gauleiter of Reichsgau Posen, and Albert Forster of Reichsgau West Prussia.

Following Operation Barbarossa of June 1941 After the 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.Leni Yahil, Ina Friedman, Ḥayah Galai, The Holocaust: the fate of European Jewry, 1932–1945, Oxford University Press US, 1991, p. 261, : confirms reaching Białystok in June 1941 and subsequent annexation of Białystok and the surrounding area, but does not detail the countiesBruno Wasser, Himmlers Raumplanung im Osten: Der Generalplan Ost in Polen, 1940–1944, Birkhäuser, 1993, p.20, : confirming that B was attached, but not incorporated ("von Ostpreußen aus verwaltet") Other Polish territories, first annexed by Soviet Union and then by Germany, was incorporated into Reichskommissariat Ostland (in the north), Reichskommissariat Ukraine (in the south) and the General Government (Distrikt Galizien in the utmost south).

Planned extension of annexation plans The Nazi government intended to continue its incorporation of pre-war Polish territory into Germany. The rump General Government region of occupied Poland already under complete German civil control was merely seen as a transitional form of government, before the area's complete future integration into the Greater German Reich (Grossdeutsches Reich).Rich, Norman (1974). Hitler's War Aims: The Establishment of the New Order. W.W. Norton & Company, p. 89. The German bureaucrats subsequently discussed various proposals for the dismemberment of the remaining territories.

Hans Frank advocated for the transformation of some or all of his province into a "Vandalengau", in honor of the East Germanic Vandal tribes who in Ancient Times had dwelt in the Vistula river basin before the Barbarian migrations. In late 1939 a sixteen-man commission was also active to chart the boundaries of a projected Reichsgau Beskidenland (named after the Beskid mountain range), which would have encompassed the areas lying west of Kraków up to the San river to the east of it.Burleigh, Michael (1988). Germany turns eastwards: a study of Ostforschung in the Third Reich. Cambridge University Press, p. 159.

Nazi Party Secretary Martin Bormann on the other hand proposed that the General Government would in the near future be turned into 3–5 Reichsgaue or Reichsobergaue, including the Galician district.Kroener, Bernhard; Müller, Rolf-Dieter; Umbreit, Hans (2003). Germany and the Second World War: Volume V/II. Oxford University Press, p. 16. Madajczyk, Czesław; Puchert, Berthold (1987). Die Okkupationspolitik Nazideutschlands in Polen 1939–1945. Akademie-Verlag, p. 102.(in German) Leaving such discussions open for the conclusion of the war, Hitler never officially adopted or implemented any of these suggestions, instead retaining the status quo of using the areas as a labor reservoir.

Administrative divisions