to enable more divisions to be sent to the Eastern Front. New defences were to be built behind the front line to contain a breakthrough until the position was restored by counter-attacks. The began the huge task of building field fortifications, which were not complete until the autumn of 1915. Background Strategic developments Plan XVII Under Plan XVII the French peacetime army was to form five field armies, with groups of Reserve divisions attached and a group of reserve divisions was to assemble on each flank, The armies were to concentrate opposite the German frontier around Épinal, Nancy and Verdun–Charleville-Mezières, with an army in reserve around Sainte-Menehould and Commercy. Since 1871 railway building had given the French General Staff sixteen lines to the German frontier against thirteen available to the German army and the French could wait until German intentions were clear. The French deployment was intended to be ready for a German offensive in Lorraine or through Belgium. It was anticipated that the Germans would use reserve troops but also expected that a large German army would be mobilised on the border with Russia, leaving the western army with sufficient troops only to advance through Belgium south of the Meuse and the Sambre rivers. French intelligence had obtained a map exercise of the German general staff of 1905, in which German troops had gone no further north than Namur and assumed that plans to besiege Belgian forts were a defensive measure against the Belgian army. A German attack from south-eastern Belgium towards Mézières and a possible offensive from Lorraine towards Verdun, Nancy and St. Dié was anticipated; the plan was an evolution from Plan XVI and made more provision for the possibility of a German offensive through Belgium. The First, Second and Third Armies were to concentrate between Épinal and Verdun opposite Alsace and Lorraine, the Fifth Army was to assemble from Montmédy to Sedan and Mézières and the Fourth Army was to be held back west of Verdun, ready to move east to attack the southern flank of a German invasion through Belgium or southwards against the northern flank of an attack through Lorraine. No formal provision was made for combined operations with the British Expeditionary Force (BEF) but joint arrangements had been made and in 1911 during the Second Moroccan Crisis the French had been told that six divisions could be expected to operate around Maubeuge. Schlieffen–Moltke