By 20 August, the Fifth Army (General Charles Lanrezac) had begun to concentrate on a 40 km (25 mi) front along the Sambre, centred on Charleroi and extending east to the Belgian fortress of Namur.

The Cavalry Corps (General André Sordet) covered the Fifth Army's left flank and the concentration of the British Expeditionary Force (BEF) at Mons.

On the morning of the 21st, the French commander-in-chief, the head of Grand Quartier Général (GQG), Joseph Joffre, communicated to Lanrezac and to the BEF that German troops were moving west.

In accordance with Plan XVII, the Third and Fourth armies further south were to move towards, respectively, Arlon and Neufchâteau, then attack German forces in Belgian Luxembourg.

The Fifth Army was ordered to cover the Meuse up to Namur and the British were to conform by moving in the general direction of Soignies, north-east of Mons.

[2] At 20:00, having reported only minor action on the X Corps front to Joffre at 19:00, Lanrezac was instructed by the latter that he had discretion to decide of the appropriate moment to start his offensive.

[6] On the French right flank, General d'Espèrey ordered I Corps troops to make movements in preparation of an offensive action.

Lanrezac was sacked by Joffre on 3 September (four days after General Pierre Ruffey, the Third Army commander) and replaced by d'Espèrey.

[8] The 1934 work by the French Fascist and writer Pierre Drieu La Rochelle, The Comedy of Charleroi, explores the author's role in the battle.