Reserve-Division) was a unit of the Imperial German Army during World War I.
The division was formed upon mobilization of the German Army in August 1914[1] as part of XVIII Reserve Corps.
The division was raised primarily in the Prussian Province of Hesse-Nassau, but one battalion of the 88th Reserve Infantry Regiment came from the Grand Duchy of Hesse, and some other troops of the division came from Westphalia and the Rhine Province.
The 21st Reserve Division fought on the Western Front, participating in the opening German offensive which led to the Allied Great Retreat and ended with the First Battle of the Marne.
Over the course of the war, other changes took place, including the formation of artillery and signals commands and a pioneer battalion.