Immediately following the German invasion of Poland, Greiser was transferred from Danzig and on 8 September was appointed "Chef der Zivilverwaltung im Militärbezirk Posen" or Chief of Civil Administration in the military district of Posen, which was annexed to the German Reich on 8 October 1939. The military administration ended and he was then appointed "Gauleiter" of the newly created Reichsgau Posen on 21 October. At the same time he was named Reich Defense Commissioner of the newly established "Wehrkreis" XXI, consisting of the new Reichsgau. On 2 November, he was also named "Reichsstatthalter" (Reich Governor) of the new territory, thereby uniting under his control the highest party and governmental offices in his jurisdiction. On 29 January 1940, the region was renamed Reichsgau Wartheland. On 30 January 1942, Greiser was promoted to SS-"Obergruppenführer".