Amid the disruption of the new rearmament plan, disorganised and poorly trained Belgian soldiers would benefit from a central position to delay contact with an invader but it would also need fortifications for defence, which were on the frontier.

[5] Alfred von Schlieffen, Chief of the Imperial German General Staff (Oberste Heeresleitung, OHL) from 1891–1906, devised plans for a decisive battle against the French army in Germany, Belgium or France.

The Schlieffen plan [sic] amounts to a critique of German strategy in 1914 since it clearly predicted the failure of Moltke’s underpowered invasion of France.

[7]The main German force tried to follow Aufmarsch I to envelop the French armies on the left (north) and press them back over the Meuse, Aisne, Somme, Oise, Marne and Seine rivers, unable to withdraw into central France.

As Lanrezac became aware of the size of the German force in Belgium and wanted to reinforce the left flank by moving to Namur, Joffre refused to allow the army front to be extended to 110 km (68 mi) and ordered Lanrezac to keep the army in a central position near Mézières, ready to oppose a German offensive from Mouzon to Namur.

On 14 August a new intelligence report showed eight German corps between Luxembourg and Liège and by the next day Joffre allowed the move of the Fifth Army north, to operate beyond the Meuse.

The Fifth Army began to move towards Namur, in the angle of the Meuse and Sambre rivers on 19 August, which required a march of 100 km (62 mi) by some units.

The capture of the 2nd Army order of battle on 7 August, convinced Joffre that the strength of the German forces on the flanks had left the centre weak and vulnerable to an offensive towards Neufchâteau and Arlon.

[17] Joffre set 14 August as the date when the First and Second armies were to invade Lorraine between Toul and Épinal, south of the German fortified area of Metz-Thionville.

The French offensive was complicated by the two armies diverging as they advanced, on difficult terrain particularly in the south, the combined fronts eventually being 150 km (93 mi) wide.

The French VII Corps with the 14th and 41st divisions, under the command of General Bonneau, advanced from Belfort to Mulhouse and Colmar 35 km (22 mi) to the north-east.

After being overrun, the Germans withdrew hastily through the Hardt forest to avoid being cut off and crossed the Rhine pursued by the French, retreating to Ensisheim, 20 km (12 mi) to the north.

Haelen was a small market town along the principal axis of advance of the German imperial army and provided a good crossing point over the River Gete.

The battle took place on 12 August between German forces, led by Georg von der Marwitz and the Belgian troops commanded by Léon de Witte.

The German pursuit was slow and Castelnau was able to occupy positions east of Nancy and extend the right wing towards the south, to regain touch with the First Army.

On 22 August in the Third army area, the V Corps attacked dug-in German troops at Longwy at 5:00 a.m. in thick fog and heavy rain, with no artillery support.

To the north the IV Corps also advanced in fog and encountered German troops dug in near Virton and was forced back also with a division routed.

[32] The French commanders were ordered by Joffre to continue the offensive on 23 August, as early as possible since his strategy depended on the success of the Third and Fourth armies.

[35] On 23 August, The German IX Corps advanced and part of the 35th Brigade got across the Mons–Condé Canal east of Nimy; reached the railway beyond in the early afternoon but was repulsed from the town.

[36] By 11:00 a.m. reports to Kluck revealed that the British were in St Ghislain and at the canal crossings to the west as far as the bridge at Pommeroeuil, with no troops east of Condé.

[41] 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.

[45] Herwig wrote that the French army did not publish formal casualty lists but that the Official History Les armées françaises dans la grande guerre gave losses of 206,515 men for August and 213,445 for September.

Longwy was surrendered by its garrison and next day British Marines and a party of the Royal Naval Air Service (RNAS) landed at Ostend; Lille and Mezières were occupied by German troops.

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.

[52] 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.

[52][b] German attacks continued on 6 September and 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.

[53] On 7 September German attacks further north drove a salient into the French defences south of Verdun at St Mihiel, which could force apart the Second and Third armies.

[57] On 5 September the Sixth Army advanced eastwards from Paris and met the German IV Reserve Corps, which had moved into the area that morning and stopped the French short of high ground north of Meaux.

Overnight the IV Reserve Corps withdrew to a better position 10 km (6.2 mi) east and French air reconnaissance observed German forces moving north to face the Sixth Army.

The BEF advanced from 6 to 8 September, crossed the Petit Morin, captured bridges over the Marne and established a bridgehead 8 km (5.0 mi) deep.