Immediately following the German invasion of Poland, Greiser was transferred from Danzig and 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 September 1939. The military administration ended the following month, and he was then appointed Gauleiter (21 October) and Reichsstatthalter für den Reichsgau Posen (26 October). On 29 January 1940, the region was renamed Reichsgau Wartheland.