The instruction of the Commander in Chief was that
Whatever the circumstances, it is the Commander in Chief's intention to advance with all forces united to the attack of the German armies. The action of the French armies will be developed in two main operations: one, on the right in the country between the wooded district of the Vosges and the Moselle below Toul; the other, on the left, north of a line Verdun–Metz. The two operations will be closely connected by forces operating on the Hauts de Meuse and in the Woëvre.
— Joffre[30]
and that to achieve this, the French armies were to concentrate, ready to attack either side of Metz–Thionville or north into Belgium, in the direction of Arlon and Neufchâteau.[31] An alternative concentration area for the Fourth and Fifth armies was specified, in case the Germans advanced through Luxembourg and Belgium but an enveloping attack west of the Meuse was not anticipated and the gap between the Fifth Army and the North Sea was covered by Territorial units and obsolete fortresses.[32]