The 9th Army was an army level command of the German Army in World War I. It was formed in September 1914 in Breslau to command troops on the southern sector of the Eastern Front. The army was dissolved on 30 July 1916, but reformed in Transylvania on 6 September 1916 for the Romanian Campaign. It was transferred to the Western Front on 19 June 1918 where it was finally dissolved on 18 September 1918.

The Second Battle of the Marne was the last major German offensive on the Western Front during the First World War. The attack failed when an Allied counterattack, supported by several hundreds of tanks, overwhelmed the Germans on their right flank, inflicting severe casualties. The German defeat marked the start of the relentless Allied advance which culminated in the Armistice with Germany about 100 days later.