On mobilization in August 1914, the 8th Army Headquarters was formed in Posen to command troops stationed in East Prussia to defend against the expected Russian attack, Plan XIX.

Initially, the Army commanded the following formations:[3] Concerned by the defeat at Gumbinnen and the continued advance of the Russian Second Army from the south, Prittwitz ordered a retreat to the Vistula, effectively abandoning East Prussia.

When he heard of this, Helmuth von Moltke, the German Army Chief of Staff, recalled Prittwitz and his deputy to Berlin.

They were replaced by Paul von Hindenburg, called out of retirement, with Erich Ludendorff as his chief of staff.

Under its new command, the Army was responsible for the victories at the Battles of Tannenberg and Masurian Lakes.