During wartime, the 21st Division, like other regular German divisions, was redesignated an infantry division. The organization of the 21st Infantry Division in 1870 at the beginning of the Franco-Prussian War was as follows:
41. Infanterie-Brigade
*Füsilier-Regiment Nr. 34
*Füsilier-Regiment Nr. 80
42. Infanterie-Brigade
*Infanterie-Regiment Nr. 81
*Infanterie-Regiment Nr. 88
Jäger-Bataillon Nr. 11
Dragoner-Regiment Nr. 5
Pre-World War I organization
German divisions underwent various organizational changes after the Franco-Prussian War. As noted above, the 21st Division was reorganized to become primarily a Hannover/Brunswick unit. The organization of the 21st Division in 1914, shortly before the outbreak of World War I, was as follows:
41. Infanterie-Brigade:
*1. Nassauisches Infanterie-Regiment Nr. 87
*2. Nassauisches Infanterie-Regiment Nr. 88
42.Infanterie-Brigade:
*Füsilier-Regiment von Gerdsdorff Nr. 80
*Infanterie-Regiment Landgraf Friedrich I. von Hessen-Kassel Nr. 81
21. Kavallerie-Brigade:
*Magdeburgisches Dragoner-Regiment Nr. 6
*Thüringisches Ulanen-Regiment Nr. 6
21. Feldartillerie-Brigade:
*1. Nassauisches Feldartillerie-Regiment Nr. 27
*2. Nassauisches Feldartillerie-Regiment Nr. 63
Order of battle on mobilization
On mobilization in August 1914 at the beginning of World War I, most divisional cavalry, including brigade headquarters, was withdrawn to form cavalry divisions or split up among divisions as reconnaissance units. Divisions received engineer companies and other support units from their higher headquarters. The 21st Division was again renamed the 21st Infantry Division and its initial wartime organization was as follows:
41. Infanterie-Brigade:
*1. Nassauisches Infanterie-Regiment Nr. 87
*2. Nassauisches Infanterie-Regiment Nr. 88
42.Infanterie-Brigade:
*Füsilier-Regiment von Gerdsdorff Nr. 80
*Infanterie-Regiment Landgraf Friedrich I. von Hessen-Kassel Nr. 81
Thüringisches Ulanen-Regiment Nr. 6
21. Feldartillerie-Brigade:
*1. Nassauisches Feldartillerie-Regiment Nr. 27
*2. Nassauisches Feldartillerie-Regiment Nr. 63
1.Kompanie/1. Nassauisches Pionier-Bataillon Nr. 21
Late World War I organization
Divisions underwent many changes during the war, with regiments moving from division to division, and some being destroyed and rebuilt. During the war, most divisions became triangular - one infantry brigade with three infantry regiments rather than two infantry brigades of two regiments. An artillery commander replaced the artillery brigade headquarters, the cavalry was further reduced, the engineer contingent was increased, and a divisional signals command was created. The 21st Infantry Division's order of battle on April 28, 1918, was as follows:
42. Infanterie-Brigade:
*Füsilier-Regiment von Gerdsdorff Nr. 80
*Infanterie-Regiment Landgraf Friedrich I. von Hessen-Kassel Nr. 81
*1. Nassauisches Infanterie-Regiment Nr. 87
2.Eskadron/Magdeburgisches Dragoner-Regiment Nr. 6
Artillerie-Kommandeur 21:
*1. Nassauisches Feldartillerie-Regiment Nr. 27
*II.Bataillon/Badisches Fußartillerie-Regiment Nr. 14
Stab 1. Nassauisches Pionier-Bataillon Nr. 21:
*1.Kompanie/1. Nassauisches Pionier-Bataillon Nr. 21
*5.Kompanie/1. Nassauisches Pionier-Bataillon Nr. 21