Reichsgau Wartheland (initially Reichsgau Posen, sometimes briefly called Warthegau) was the name given by Nazi German government to the largest subdivision of the territory of Greater Poland which was directly incorporated into the German Reich after defeating the Polish army in 1939. Its capital was Poznań (Posen).
The name "Wartheland" refers to the German name given to the Warta River. Historically, the main parts of the area had been annexed by the Kingdom of Prussia from 1793 until 1807. From 1815–1849 the territory was within the autonomous Grand Duchy of Posen, after which it was renamed the Province of Posen. The Poles regained independence in 1918–1919 following World War I.
The territory was inhabited by Poles and a German minority (16.7 % of total population in 1921). During World War II 630,000 Poles and Jews were expelled from the territory into the occupied General Government (more than 70,000 from Poznań alone) in actions called the Kleine Planung.
After the invasion of Poland, the conquered territory was partitioned among four different Reichsgaue and the General Government area further east. Militärbezirk Posen was created in September 1939 and as 'Reichsgau Posen annexed to Germany on October 8, 1939, with SS Obergruppenfuhrer Arthur Greiser as the first and only Gauleiter. The name Reichsgau Wartheland was introduced on January 29, 1940.
The Wehrmacht established Wehrkreis XXI based at Poznań. This Wehrkreis was under the command of General de Artillerie Walter Petsel, and its primary operational unit was the XXXXVIII Panzer Korps. Poznań was responsible for the Militärische Unterregion-Hauptsitze at Poznań, Leszno, Inowrocław, Włocławek, Kalisz, and Łódź. It maintained training areas at Sieradz and Biedrusko.
In the Wartheland, the Nazis' goal was complete "Germanization", or political, cultural, social, and economic assimilation of the territory into the German Reich. In pursuit of this goal, the installed bureaucracy renamed streets and cities and seized tens of thousands of Polish enterprises, from large industrial firms to small shops, without payment to the owners.
The Germanization of the annexed lands also included an ambitious program to resettle Germans from the Baltic and other regions on farms and other homes formerly occupied by Poles and Jews. By the end of 1940, the SS had expelled 325,000 Poles and Jews from the Wartheland and the Polish Corridor and transported them to the General Government, confiscating their belongings. Many elderly people and children died en route or in makeshift transit camps such as those in the towns of Potulice, Smukal, and Toruń. In 1941, the Nazis expelled a further 45,000 people.