It would take the French three weeks to fully mobilise her forces and up until the 15th August the two General Staffs followed their pre-war plans.
Advancing into Belgium the German 2nd Army arrived in front of Liège on the 6th August and having taken the town began its siege of the outlying forts.
Initially von Bülow had attempted to take these by frontal attacks, but he was beaten off with heavy losses and eventually settled on using the heavier calibre guns — which had to be brought up to the front.
This stand by the Belgian garrison cost the Germans four or five days on their timetable.
Despite the Germans evident strength in front of Liège Joffre maintained that their primary forces were concentrated around Metz (in German Lorraine it should be recalled). It was inconceivable that the Germans could be strong on both fronts.
Either they would continue westwards should Liège fall or they would pivot on Metz striking at the French left flank.
On the 8th August 1914 Joffre issued his General Instruction No 1.
In front of the French 1re Armée and 2e Armée the Germans appeared to have mustered no more than perhaps six Corps of infantry. Their major strength was gathered around Metz whilst elements of five Corps had entered Belgium and were engaged with Belgian forces.
With all forces united the French would strike against the Rhine and the German right flank.
To facilitate this attack a detachment from the 1re Armée — 7e Corps d’armée (CA) — crossed the frontier on the 6th August. They reached Mulhouse (Mulhäusen) but were forced to retire in the face of superior forces.
Thus, on the 14th August an augmented strike force now called the Army of Alsace under Général Pau entered Alsace on the same day as the 1re Armée and 2e Armée commenced their liberation of Lorraine.
On the 10th August the first combat of the war on French soil had taken place at the village of Mangiennes near the Belgian border and only thirty kilometres north of Verdun.
On the 15th August news was received that the French 1er CA (5e Armée) had encountered von Richthofen’s cavalry corps at Dinant, and the Belgians reported 200,000 Germans crossing the Meuse near Liège.
Général Lanrezac commanding the French 5e Armée finally managed to convince Joffre that the Belgian frontier needed protecting (An eventuality already covered by Plan XVII). The 5e Armée duly moved westwards towards Philippeville and Chimay. Although they were reinforced by the 3e and 10e CA they lost the 2e CA to de Langle de Cary’s 4e Armée.
The left flank of the operation would be covered by the British Expeditionary Force (BEF) which had arrived in France and was marching towards its designated concentration area at Le Cateau.
To Joffre it seemed that the Germans were in the process of crossing Belgium with one force whilst concentrating another in the area of Thionville and the Belgian Ardennes. In front of Metz they appeared to be on the defensive.
Now was the moment to strike with the 3e Armée and 4e Armée up through Belgian Luxembourg and the Duchy itself to take the Germans in the flank. This was the centre of their forces but if the French moved with speed the German forces in Belgium would not have sufficient time to swing to the south to face the threat.
Once the Germans were broken, Joffre would have the choice of rolling them up from either flank.
On the 20th August the 3e Armée (Général Ruffey) was ordered towards Arlon and to counter attack any attempt made to gain the right flank of the 4e Armée.
De Langle de Cary was ordered to send a strong advanced guard that night towards Tintigny to allow the crossing of the Semois River with his main force in the direction of Neufchâteau.
With Lanrezac’s 5e Armée already well advanced into the triangle between the rivers Sambre and Meuse any failure by this operation to bring the other armies into line, would make Lanrezac’s position untenable.
By this time, apart from the delay at Liège, the Germans were reasonably on target. To avoid violating Dutch neutrality von Kluck’s 1st Army had passed through a narrow corridor at Aachen and had formed up on von Bülow’s right flank. Their job was to deal with the Belgian Army and force it away from Antwerp. In this they failed and were forced to detail troops to ensure that the Belgians remained there.
The 3rd Army under von Hausen was at Namur and the 4th and 5th Armies were completing their initial advance into the Belgian Ardennes.
Like an enormous barrier swinging down across Belgium the five German armies were pivoting on Thionville.
On the 19th August on their Lorraine front (Lothringen in German) Crown Prince Ruprecht of Bavaria had stopped the French advance in its tracks. and had swung his eight corps against the French six.
The following day his heavy artillery wreaked havoc amongst de Castelnau’s 2e Armée and it was fortunate that the French 20e CA was not only fighting on home ground but was commanded by Général Ferdinand Foch.
As the British Expeditionary Force approached the Belgian town of Mons to form the left flank of the Allied forces, the campaign in Lorraine had been gutted and the campaign in the Ardennes was marching to disaster in the forests of Wallonia.