[3] The division was recruited in the formerly independent Duchy of Nassau and the Electorate of Hesse, which had been incorporated into Prussia after the Austro-Prussian War, and in the city of Frankfurt am Main.
The division was disbanded in 1919 during the demobilization of the German Army after World War I.
[4] In World War I in 1914, the 21st Infantry Division fought in the Allied Great Retreat, including the First Battle of the Marne, and in the Race to the Sea.
It served in the German 1918 Spring Offensive, including the Second Battle of the Somme.
Divisions received engineer companies and other support units from their higher headquarters.