On the evening of the 4th August 1914 King Albert I gave permission for both French and British forces to cross the Belgian frontier in order to assist the Belgian army to repel the German invaders.
The French Cavalry Corps under Général Sordet had already been deployed near Mezières but with strict instructions not to cross into Belgium (In fact all French border patrols had been warned that they were on no account to cross any frontier - it was extremely important that the Germans were seen as the aggressors).
On the 5th Sordet received his orders to move his three cavalry divisions into Belgium and the march north towards Liège commenced the following day.
Sordet had a fourfold mission:
French Military Intelligence insisted that the mass of the German army was gathering behind Metz and Thionville. From there it was assumed the Germans would either launch an attack into Lorraine or a flanking movement via Belgium.
For the next two weeks Sordet's force would be ordered to ride out as far as Liège (where the Germans were giving siege to the fortresses), then back to Neufchâteau in the Belgian Ardennes, then out towards Bruxelles before being brought back once more - this time alongside Lanrezac's 5th Army near Charleroi.
See linked map
In blisteringly hot weather they had been asked to ride hundreds of kilometres, fatiguing their horses and all to no avail. In fact they had moved too quickly. Whilst the German 1st and 2nd Armies had indeed moved into Belgium as the right fist of the German movement, the 3rd and 4th providing the forearm had yet to commence their part in the wheel.
It should be understood that French military thinking had for some time allowed for the possibility of a German advance through Belgium, but this was considered to aid the French Plan XVII which would punch through into Alsace and Lorraine. The more Germans on the French left, the less directly in front, went the theory.
Even before the war had started there had been voices in France that warned of a German flanking movement through Belgium and one of those voices was that of Général Lanrezac. Throughout the opening manoeuvres of the war he expressed his concerns to his Commander in Chief; Général Joffre that the Germans were intent on more than taking out a few Belgian fortresses.
Up until the 15th Joffre was deaf to his theories. That night however Lanrezac received a communication stating that: the enemy appear to be making their principle thrust to the north of Givet (just inside France and south of Dinant). There is another force which seems to be marching towards the line of Sedan-Montmédy-Damvillers.
On the 10th August an encounter battle had been fought near the border at Mangiennes so there was little doubt that the Germans were probing the French defences.Mangiennes: 10 August 1914
Having argued for some time that he should be moving his army towards the north-west Lanrezac was finally given permission to march to the Sambre.
Even at this stage however French Intelligence had little idea as to what the Germans were planning. On the 17th August Field Marshal French arrived at Joffre's GQG (GHQ) at Vitry to give details of the British Expeditionary Force's deployment on Lanrezac's left flank. Joffre assured French that his right wing would be covered by that of the 5th Army.
Believing that all was well French drove on to Rethel to meet with Lanrezac (who was noted for his fiery temper and contempt for the BEF). The meeting has become an anecdote of the war. Having just been briefed by Joffre, French pointed to a map of the area and asked where Lanrezac thought the Germans would attempt to cross the Meuse.
Neither man spoke the other's language but the atmosphere became frosty when a stunned Lanrezac replied that the German's had crossed the Meuse 24 hours previously. Why? Asked the Field Marshal evidently wanting to know what Lanrezac thought their intentions were. The Frenchman, perhaps already irritated by GQG's deafness and evident lack of up to date information, replied as sarcastically as he could: Let's see, perhaps they have come to fish.
Franco-British relations plummeted from there on, and when a few days later Lanrezac retired his army in the face of a three pronged German offensive (The Battle of Charleroi) he made no attempt to warn French that he was doing so, leaving the BEF exposed at Mons.
To Joffre at GQG 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 3rd and 4th Armies 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.