At the outbreak of World War I, the Belgian army was in the middle of a reorganisation. Historically, Belgium had a reputation for neglecting its military. During the 19th century, military reform had been a major political issue as successive governments remained unsure of whether the signatory nations of the 1839 Treaty of London would intervene to guarantee Belgian neutrality if the country were invaded. Belgian politicians were also aware of the rapid expansion of French and German armies during the period. From the 1880s onwards, the Belgian government embarked on an ambitious series of fortress construction but failed to expand or reform the military itself. In 1902, the regular army stood at just 42,800 men with a potential post-mobilisation strength of 180,000.
Further east the Third Army was forced back to the west of Verdun as German attacks were made on the Meuse Heights to the south-east but managed to maintain contact with Verdun and the Fourth Army to the west. German attacks against the Second Army south of Verdun from 5 September almost forced the French to retreat but on 8 September the crisis eased. By 10 September the German armies west of Verdun were retreating towards the Aisne and the Franco-British were following up, collecting stragglers and equipment. On 12 September Joffre ordered an outflanking move to the west and an attack northwards by the Third Army to cut off the German retreat. The pursuit was too slow and on 14 September the German armes had dug in north of the Aisne and the Allies met trench lines rather than rearguards. Frontal attacks by the Ninth, Fifth and Sixth armies were repulsed on 15–16 September, which led Joffre to begin the transfer of the Second Army west to the left flank of the Sixth Army, the first phase of the Race to the Sea, a series of operations to outflank the German armies, which from 17 September to 17–19 October moved the opposing armies through Picardy and Flanders to the North Sea coast.
Joffre used the railways which had transported French troops to the German frontier to move troops back from Lorraine and Alsace to form a new Sixth Army under General Michel-Joseph Maunoury with nine divisions and two cavalry divisions. By 10 September twenty divisions and three cavalry divisions had been moved west from the German border to the French centre and left and the balance of force between the German 1st–3rd armies and the Third, Fourth, Ninth, Fifth armies, the BEF and Sixth Army had changed to 44:56 divisions. Late on 4 September Joffre ordered the Sixth Army to attack eastwards over the Ourcq towards Château Thierry as the BEF advanced towards Montmirail and the Fifth Army attacked northwards, with its right flank protected by the Ninth Army along the St. Gond marshes. The French First–Fourth armies to the east were to resist the attacks of the German 5th–7th armies between Verdun and Toul and repulse an enveloping attack on the defences south of Nancy from the north. The 6th and 7th armies were reinforced by heavy artillery from Metz and attacked again on 4 September along the Moselle.
On 7 September German attacks further north drove a salient into the French defences south of Verdun at St. Mihiel, which threatened to separate the Second and Third armies. At Nancy, part of the 59th Reserve Division retreated from the height of St. Geneviève, which overlooked the Grand Couronné to the north-west of Nancy, exposing the left flank of the Second Army and Nancy to envelopment. Castelnau prepared to withdraw and abandon Nancy but was circumvented by the Second Army staff, who contacted Joffre. Castelnau was ordered to maintain the defence of the Grand Couronné for another 24 hours. The French abandonment of the height of St. Geneviève went unnoticed by the Germans, who had retired during the afternoon and the height was reoccupied. German attacks continued until the morning of 8 September but then became less powerful as Moltke began to withdraw troops to the right flank of the German armies. Moltke sent Major Roeder to the 6th Army with orders to end the offensive and prepare to retire to the frontier; only at this point did Rupprecht find out that the armies near Paris were under severe pressure. The attacks by the 6th Army diminished and on 10 September the army began to withdraw towards the frontier. On the 13 September, Pont-à-Mousson and Lunéville were recaptured by the French unopposed and the French armies closed up to the Seille river, where the front stabilized until 1918.
The German offensive began during the night of 3 September against the fortifications of the Grand Couronné, either side of Nancy, which pushed back the 2nd Group of Reserve Divisions, comprising the 59th, 68th and 70th Reserve Divisions under General Léon Durand, to the north and the XX Corps of General Balfourier to the south, by the evening of 4 September. In the afternoon of 5 September Castelnau telegraphed to Joffre that he proposed to evacuate Nancy, to preserve the fighting power of the army. Next day Joffre replied that the Second Army was to hold the area east of Nancy if at all possible and only then retire to a line from the Forest of Haye to Saffais, Belchamp and Borville. The civilian authorities in the city had begun preparations for an evacuation but the troops on the Grand Couronné repulsed German attacks on the right flank, during 5 September. The Reserve divisions were only pushed back a short distance on the front to the east and north of Nancy. An attempt by Moltke to withdraw troops from the 6th Army, to join a new 7th Army being formed for operations on the Oise failed when Rupprecht and Dellmensingen were backed by the Emperor who was at the 6th Army headquarters. German attacks continued on 6 September and the XX Corps conducted a counter-attack which gave the defenders a short period to recuperate but the troops of the 2nd Group of Reserve Divisions, east and north of Nancy, began to give way.
By 4 September the First and Second armies had slowed the advance of the 7th and 6th armies west of St. Dié and east of Nancy, from where the Second Army had withdrawn its left flank, to face north between Nancy and Toul. A gap between the left of the Second Army and the right of the Third Army at Verdun, which faced north-west, on a line towards Revigny against the 5th Army advance, west of the Meuse between Varennes and St. Ménéhould. The Fourth Army had withdrawn to Sermaize, west to the Marne at Vitry le François and then across the river to Sompons, against the 4th Army, which had advanced from Rethel, to Suippes and the west of Chalons. The new Ninth Army held a line from Mailly against the 3rd Army, which had advanced from Mézières, over the Vesle and the Marne west of Chalons. The 2nd Army had advanced from Marle on the Serre, across the Aisne and the Vesle, between Reims and Fismes to Montmort, north of the junction of the Ninth and Fifth armies at Sezanne. The Fifth Army and the BEF had withdrawn south of the Oise, Serre, Aisne and Ourq, pursued by the 2nd Army on a line from Guise to Laon, Vailly and Dormans and by the 1st Army from Montdidier, towards Compiègne and then south-east towards Montmirail. The new French Sixth Army, linked with the left of the BEF, west of the Marne at Meaux, to Pontiose north of Paris. French garrisons were besieged at Strasbourg, Metz, Thionville, Longwy, Montmédy and Maubeuge. The Belgian army was invested at Antwerp in the National redoubt and at fortress troops continued the defence of the Liège forts.
