On mobilisation, XX Corps was assigned to the 8th Army to defend East Prussia while the rest of the Army executed the Schlieffen Plan offensive in August 1914. It took part in the battles of Gumbinnen, Tannenberg and 1st Masurian Lakes. Immediately after 1st Masurian Lakes it joined 9th Army in Lower Silesia where it fought at the Battle of the Vistula River. By May 1915 it was back with 8th Army and on 26 May the commander of XX Corps, General der Artillerie Friedrich von Scholtz, was made commander of the Army, while retaining simultaneous command of his Corps. On 29 September 1915, the headquarters of 8th Army was dissolved. On 18 September 1915, XX Corps was upgraded to form "Armee-Gruppe" Scholtz, later "Armee-Abteilung" Scholtz, as part of the Army of the Niemen.