Armee-Abteilung Lauenstein (Army Detachment Lauenstein) was formed by upgrading XXXIX Reserve Corps of 10th Army on 22 April 1915. It was named for its commander, Generalleutnant Otto von Lauenstein, who retained simultaneous command of XXXIX Reserve Corps. It was directly under the command of OB East.
Due to its success, it was continuously reinforced until it was raised to the status of an army as the Army of the Niemen. Generalleutnant von Lauenstein remained as commander of XXXIX Reserve Corps.
Army of the Niemen
The Army of the Niemen was formed on 26 May 1915 to control the troops in Courland. The former commander of the 8th Army, General der InfanterieOtto von Below, along with his Chief of Staff, Generalmajor von Böckmann, assumed command.