After the French defeat in the Franco-Prussian War 1870–1871, from 1874–1880, General Raymond Adolphe Séré de Rivières (20 May 1815 – 16 February 1895) oversaw the construction of the Séré de Rivières system, a line of fortresses 65 km (40 mi) long from Belfort to Épinal and another line of similar length from Toul to Verdun, about 40 km (25 mi) back from the frontier.

A second series of fortifications, to prevent the main line being outflanked, was built in the south, from Langres to Dijon and in the north from La Fère to Rheims and from Valenciennes to Maubeuge, although for financial reasons these defences were incomplete in 1914.

The completion of the fortress lines between Belfort and Verdun in the late 1880s and railway building from the interior to the border, then gave the French army the means to contemplate a defensive-offensive strategy, in which a German attack would be repulsed and then followed up by a counter-attack.

In August 1891, Plan XI was completed, with an option for an offensive as well as a defensive strategy from the start, to exploit the opportunity created by the improvement in relations between the Third Republic and the Russian Empire.

Michel thought that the Germans would make their main effort in central Belgium and that covering a longer front would need the organisation of French reserve units and integration with the active army.

The council rejected his view in 1911, which caused Michel to resign as he considered that deployment from Belfort to Mézières and an offensive towards Antwerp, Brussels and Namur as the only possible way to respond.

[5] On 9 January 1912, the Conseil supérieur de la guerre agreed that the French army could enter Belgium but only when news had arrived that the Germans had already done so.

The council also considered industrial mobilisation and the slow speed of the development of heavy artillery and agreed to increase the ammunition stock from 1,280 shells-per-gun to 1,500.

An offensive strategy required an adequate field of operations and Belgium was the only place where the terrain was suitable but Belgian and British sensitivities remained paramount.

[5] French politicians feared that violating Belgian sovereignty would force Belgium to join Germany in the event of war and cause Britain to withdraw from its military commitments.

[6] Despite the matter of Belgian neutrality, Joffre remained favourable to an offensive (rather than defensive-offensive) strategy and the benefit of compelling the Germans to fight against both France and Russia.

[7] For political purposes, Joffre hid his intentions from the French government, the Plan XVII included deployments near southern Belgium but did not explicitly prepare to cross the Belgian frontier.

[9] The document was not a campaign plan but it contained a statement that the Germans were expected to concentrate the bulk of their army on the Franco-German border and might cross before French operations could begin.

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.

[16] The French attack into Alsace-Lorraine was defeated because of inadequate tactics, a lack of artillery-infantry co-operation and by the fighting capacity of the German armies, which inflicted huge numbers of casualties.

[13] The attacks in southern Belgium were conducted with negligible reconnaissance or artillery support and were repulsed without preventing the western manoeuvre of the German armies in the north.

[23] The reality was that many of the French casualties were said to have come from an excess of offensive vigour and on 23 August, General Pierre Ruffey concluded that the infantry had attacked without artillery preparation or supporting-fire during the battle.

[22] Early on 24 August, Joffre ordered a withdrawal to a line from Verdun to Mézières and Maubeuge and began to transfer troops from the east, opposite the German border, to the western flank.