By 20 August, the Fifth Army had begun to concentrate on a front along the Sambre, centred on Charleroi and extending east to the Belgian fortress of Namur. On the left flank, the Sordet Cavalry Corps linked the Fifth Army to the British Expeditionary Force (BEF) at Mons. General Joseph Joffre ordered Lanrezac to attack across the Sambre but this attack was forestalled by the German 2nd Army on the morning of 21 August, which crossed the Sambre, establishing two bridgeheads which the French, lacking artillery, were unable to reduce. Bülow attacked again on 22 August with three corps against the entire Fifth Army front. Fighting continued on 23 August, when the French centre around Charleroi began to fall back. The German 3rd Army crossed the Meuse and launched an attack against the French right flank, held by I Corps (General Louis Franchet d'Esperey). The French stopped the German advance and delivered a counter-attack. The Fifth Army was confronted by the German 3rd and 2nd armies from the east and the north. Before the Fifth Army could attack over the Sambre the 2nd Army attacked at the Battle of Charleroi and at Namur on 21 August. The 3rd Army crossed the Meuse and attacked the French right flank and on 23 August, the Fifth Army began a retirement southwards to avoid encirclement.