The Reichsgau Wartheland (initially Reichsgau Posen, sometimes called Warthegau) was a Nazi German Reichsgau formed from Polish territory annexed in 1939. It comprised the Greater Poland and adjacent areas, and only in part matched the area of the similarly named pre-Versailles Prussian province of Posen. The name was initially derived from the capital city, Posen (Poznań), and later from the main river, Warthe (Warta).