On 8 August, Bonneau cautiously continued the advance and occupied Mulhouse, shortly after its German defenders had left. the First Army commander General Auguste Dubail preferred to dig in and wait for mobilization of the army to be completed but Joffre ordered the advance to continue. In the early morning of 9 August, parts of the XIV and XV Corps of the German 7th Army arrived from Strasbourg and counter-attacked at Cernay. The German infantry then emerged from the Hardt forest and advanced into the east side of the city. French command broke down and the defenders fought isolated actions before pulling back as best they could, as the German attackers exploited their advantage. Mulhouse was recaptured on 10 August and Bonneau withdrew towards Belfort. Further north, the French XXI Corps made costly attacks on mountain passes and were forced back from Badonviller and Lagarde, where the 6th army took 2,500 French prisoners and eight guns; civilians were accused of attacking German troops and subjected to reprisals.
The deployment plan for the western armies Westheer, allocated to the 7th Army (Generaloberst Josias von Heeringen) the XIV and XV corps, the XIV Reserve Corps and the 60th Landwehr Brigade, to deploy from Strasbourg to Mulhouse and Freiburg-im-Breisgau and the command of fortresses at Strasbourg and Neuf-Brisach. The 1st and 2nd Bavarian brigades, 55th Landwehr Brigade, Landwehr Regiment 110 and a battery of heavy field howitzers were also added to the army under the provisional command of the XIV Corps commander. In 1914, the XIV Corps near the Swiss border, guarded the frontier with the 58th Brigade and XV Corps guarded the border from Donon to the Rheinkopf, with several infantry regiments and Jäger battalions, with some artillery and cavalry. The mobilisation and deployment was completed from 8–13 August but the German troops were concentrated further north than anticipated, to be ready to meet a French offensive from Belfort with concentric attacks from the north and east.
On 8 August, Bonneau cautiously continued the advance and occupied Mulhouse, shortly after its German defenders had left. the First Army commander General Auguste Dubail preferred to dig in and wait for mobilization of the army to be completed but Joffre ordered the advance to continue. In the early morning of 9 August, parts of the XIV and XV Corps of the German 7th Army arrived from Strasbourg and counter-attacked at Cernay. The German infantry then emerged from the Hardt forest and advanced into the east side of the city. French command broke down and the defenders fought isolated actions before pulling back as best they could, as the German attackers exploited their advantage. Mulhouse was recaptured on 10 August and Bonneau withdrew towards Belfort. Further north, the French XXI Corps made costly attacks on mountain passes and were forced back from Badonviller and Lagarde, where the 6th army took 2,500 French prisoners and eight guns; civilians were accused of attacking German troops and subjected to reprisals.
A few border skirmishes took place after the declaration of war and German reconnaissance patrols found that the French had a chain of frontier posts, supported by larger fortified positions further back; after 5 August more patrols were sent out as French activity increased. French troops advanced from Gérardmer to the Schlucht Pass, where the Germans retreated and blew up the tunnel.[10] Joffre had directed the First and Second armies to engage as many German divisions as possible, in order to assist French forces operating further north. The French VII Corps with the 14th and 41st divisions, under the command of General Bonneau, advanced from Belfort to Mulhouse and Colmar 35 kilometres (22 mi) to the north-east. The French advance was hampered by the breakdown of the supply service and many delays but seized the border town of Altkirch 15 kilometres (9.3 mi) south of Mulhouse with a bayonet charge.
At midnight on 31 July – 1 August, the German government sent an ultimatum to Russia and announced a state of "Kriegsgefahr" during the day; the Turkish government ordered mobilisation and the London Stock Exchange closed. On 1 August the British government ordered the mobilisation of the navy, the German government ordered general mobilisation and declared war on Russia. Hostilities commenced on the Polish frontier, the French government ordered general mobilisation and next day the German government sent an ultimatum to Belgium, demanding passage through Belgian territory, as German troops crossed the frontier of Luxembourg. Military operations began on the French frontier, Libau was bombarded by a German light cruiser SMS Augsburg and the British government guaranteed naval protection for French coasts. On 3 August the Belgian Government refused German demands and the British Government guaranteed military support to Belgium, should Germany invade. Germany declared war on France, the British government ordered general mobilisation and Italy declared neutrality. On 4 August the British government sent an ultimatum to Germany and declared war on Germany at midnight on 4–5 August, Central European Time. Belgium severed diplomatic relations with Germany and Germany declared war on Belgium. German troops crossed the Belgian frontier and attacked Liège.
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.
Under Plan XVII, the French peacetime army was to form five field armies of c. 2,000,000 men, with "Groups of Reserve Divisions" attached to each army and a Group of Reserve Divisions on each of the extreme flanks. The armies were to concentrate opposite the German frontier around Épinal, Nancy and Verdun–Mezières, with an army in reserve around Ste. Ménéhould 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.
