The armies of France and Germany had completed their mobilisation, the French with Plan XVII, to conduct an offensive through Lorraine and Alsace into Germany and the Germans with Aufmarsch II West, for an offensive in the north through Luxembourg and Belgium into France, supplemented with attacks in the south to prevent the French from transferring troops to the greater threat in the north.

The likelihood of a German invasion did not lead the Belgian government to see France and Britain as potential allies nor did it intend to do more than protect its independence.

Amid the disruption of the new rearmament plan, the disorganised and poorly trained Belgian soldiers would benefit from a central position to delay contact with an invader but it would also need fortifications for defence, which were on the frontier.

Belgian plans became a compromise in which the field army concentrated behind the Gete river, with two divisions forward at Liège and Namur.

German planning was determined by numerical inferiority, the speed of mobilisation and concentration and the effect of the vast increase of the power of modern weapons.

Frontal attacks were expected to be costly and protracted, leading to limited success, particularly after the French and Russians modernised their fortifications on the frontiers with Germany.

Alfred von Schlieffen Chief of the Imperial German General Staff (Oberste Heeresleitung, OHL) from 1891–1906, devised a plan to evade the French frontier fortifications with an offensive on the northern flank with a local numerical superiority.

On 11 August a French night attack was repulsed but events in the Vosges led to the I Bavarian Corps moving quickly to Eyweiler and Sieweiler.

One corps and the Second Group of Reserve Divisions advanced slowly towards Morhange in echelon, as a flank guard against a German attack from Metz.

[8] Despite warnings from Joffre against divergence, the army was required to advance towards the Vosges passes to the south-east, eastwards towards Donon and north-east towards Sarrebourg.

German troops withdrew during the day, Donon was captured and on the left flank, an advance of 10–12 km (6.2–7.5 mi) was made.

The German pursuit was slow and Castelnau was able to occupy positions east of Nancy and extend the right wing towards the south, to regain touch with the First Army.