Armee-Korps) was a corps level command of the German Army, before and during World War I.

[5] The 25 peacetime Corps of the German Army (Guards, I - XXI, I - III Bavarian) had a reasonably standardised organisation.

Divisions received engineer companies and other support units from the Corps headquarters.

On mobilisation, XXI Corps was assigned to the predominantly Bavarian 6th Army forming part of the left wing of the forces for the Schlieffen Plan offensive in August 1914 on the Western Front.

By 1915 it was on the Eastern Front where it took part in the siege of Kovno and the battles on the Neman River and at Vilnius.