The XI Army Corps / XI AK (German: XI. Armee-Korps) was a corps level command of the Prussian and German Armies before and during World War I. XI Corps was one of three formed in the aftermath of the Austro-Prussian War (the others being IX Corps and X Corps). The Corps was formed in October 1866 with headquarters in Kassel. The catchment area included the newly annexed Province of Hesse-Nassau and the Thuringian principalities (Saxe-Weimar, Saxe-Meiningen, Saxe-Coburg and Gotha and Waldeck). During the Franco-Prussian War it was assigned to the 3rd Army.