On the 20th August the French 3rd Army (Général Ruffey) was ordered across the border towards Arlon and to counter attack any attempt made to gain the right flank of the 4th Army.
The 4th Army under Général 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.
The Ardennes is a densely forested area with few villages or towns to supply provisions. The road network is poor and the rivers and streams run east-west forming natural obstacles to the French who were trying to advance south-north. Whilst the terrain did not particularly favour either side, better reconnaissance on their behalf meant that German commanders had a more accurate idea as to where the enemy were.
By the 20th August Lanrezac's 5th Army was positioned with two Corps on the Sambre and his third as a flank guard between Givet and Namur.
Joffre seems to have stubbornly ignored all reports from Lanrezac to the effect that the Germans were continuing their march westwards.
At this stage the German 3rd, 4th and 5th Armies had begun their advance into Belgium. The 3rd Army was marching in the direction of Namur, the 4th Army towards Givet and the 5th Army towards Arlon.
Although the 4th army under Langle de Cary was moving slowly into position ready for its advance into Belgium, Lanrezac's right flank remained highly exposed as he was some 50 kilometres further forward.
That evening Joffre confirmed the advance into Belgium against the German flank.
The 2è CA (Corps d'Armée - 2nd Corps) would be the advanced guard of the 4th Army.
The War Diary of the 2è CA mentions the fact that:
Ils n'est signalé dans les bois du Luxembourg Belge que des patrouilles de cavalerie. La direction donnée au CA après le passage de la frontière belge est Nord.
Only cavalry patrols have been reported within the Luxembourg (Belgium) forests. Having crossed the frontier the Corps will move north.
At 0345 hours on the 21st August they received a modified set of instructions which proscribed that the 4è DI (Division d'Infanterie - 4th Infantry Division) would provide the advanced guard crossing the border to the north of Montmédy and were to camp at Villers la Loue with its lead detachments. The remainder of the division would be stretched between there and Sommethonne.
The 3è DI would occupy the area around Montmédy whilst the Divisional Cavalry the 19è Chasseurs � Cheval would be sent ahead to scout Bellefontaine inside the Belgian border.
By the afternoon the Corps had received yet more instructions, ordering it to ensure that its advanced guard reached Bellefontaine with the main body at Meix devant Virton.
In theory the 4è DI was supposed to advance with the 7è Brigade (7th Brigade) in the lead (their advanced guard moving to Bellefontaine) and the 87è billeting in and around Meix.
Opposing them, the German 5th Army (Commanded by the Prussian Crown Prince Wilhelm) had recognised the danger as they marched on Virton and requested the 4th Army (Duke Albrecht of Württemberg) to pivot their left flank 6th Army Corps southwards.