Whilst von Schlieffen was putting his plan of attack together the French High Command was also preparing for a possible (inevitable ?) war with Germany.
Just before the turn of the century Plan XIV (the 14th plan since 1875) based itself on the imposing system of fortresses that General Séré de Rivières had constructed along France's eastern frontier. The gaps between these fortresses (Such as Longwy, Verdun, Epinal and Belfort) were covered by a system of smaller forts.
If the Germans attacked, the French would defend and wear them down. Although Germany was the more populous of the two nations and would therefore be able to field a larger army, the French were only required to resist long enough for Russia to come to her aid. The magical figure considered by all sides being six weeks for the Russian army to mobilise.
A dozen years later, fears were already growing that the Germans might try swinging through Belgium. It was a radical solution but why after all should the Germans throw themselves against France's impregnable fortresses if they could go around them. General Michel, then Chief of Staff wrote that he considered that an entire war could be fought on Belgian territory.
It would be prudent to place two thirds of the French Army along the Belgian border in readiness. This plan required placing the reservists into the front line and was thus set aside. Michel was ousted for having ideas that were considered to be bordering on the ridiculous. He was replaced by General Joseph Joffre.
Initially Joffre felt that the options open to him in assaulting Lorraine and Alsace were too limiting. He proposed extending the French assault through the tip of Belgium as well as Luxembourg. The political implications of being the first to violate Belgian neutrality however outweighed any military gain. In 1912 he began working on Plan XVII which worked at coordinating the rail systems to bring the troops to the front even faster than before. Defence was no longer a dominating issue.
The intention of the Commander in Chief is to advance with all forces united to the attack on the German armies.
Joffre believed that Germany would be able to mobilise its Reserves far faster than France. He reasoned therefore that France had to defeat the German standing army with her own as quickly as possible. To do that he had to engage the Germans immediately on the outbreak of hostilities.
Apart from wanting to take back Alsace Lorraine, this was almost certainly going to be the area where the German forces would be concentrated. The decisive battle had to be fought as the Russians brought their forces into play (which they had agreed to do before they had fully completed their own mobilisation in order to put pressure on the Germans).
The 1st Army (280,000 men) at Belfort would penetrate northern Alsace and march on Sarrebourg with its right wing on Colmar.
The 2nd Army (180,000 men) at Nancy would strike into German occupied Lorraine in the general direction of Saarbr�cken.
The 3rd Army at Verdun (200,000 men) would ensure the smooth liaison between the two wings of the French forces attacking north of a line between Verdun and Metz.
The 5th Army (240,000 men) would form up between Verdun and the Belgian frontier with a cavalry corps on its left. It would either attack alongside the 3rd Army or, in the event of a German invasion of Belgium, advance northwards to meet that threat.
The 4th Army (160,000 men) would be placed in reserve in the area of Bar le Duc. According to eventualities it would either follow behind the 2nd and 3rd armies or, in the event of hostilities developing along the Belgian frontier, between the 3rd and the 5th armies.
The French believed that even if the Germans violated Belgian neutrality it would not be to any great extent. Perhaps the biggest mistake that Joffre's intelligence staff made was in refusing to accept that the Germans were capable of putting their 1st line Reserves into the field from the start and moreover, alongside the Active units. The idea of using reservists as active soldiers was something that the French could not conceive - reserves were for, well, the reserve.
Without realising it Joffre was about to play a willing role in the Schlieffen plan - something that its author had never conceded as being likely. He would advance directly into the German army detailed to hold and then slowly give way, sucking the French further and further away from Paris.