On 15 August, the Second Army reported that German long-range artillery had been able to bombard the French artillery and infantry undisturbed and that dug-in German infantry had inflicted many casualties on the French as they attacked. The Second Army had to attack methodically after artillery preparation but managed to push back the German defenders. Intelligence reports identified a main line of resistance of the German 6th Army and 7th Army, which had been combined under the command of Crown Prince Rupprecht of Bavaria, close to the advanced French troops and that a counter-offensive was imminent. On 16 August, the Germans opposed the advance with long-range artillery fire and on 17 August, the First Army reinforced the advance on Sarrebourg. When the Germans were found to have left the city Joffre ordered the Second Army to incline further to the north, which had the effect of increasing the divergence of the French armies. A German counter-attack on 20 August forced separate battles on the French armies, which were defeated and forced to retreat in disorder. The German pursuit was slow and Castelnau was able to occupy positions east of Nancy and extend the right wing towards the south, to regain touch with the First Army. During 22 August, the right flank was attacked and driven back from the position where the offensive had begun on 14 August. The First Army withdrew but managed to maintain contact with the Second Army and on 24 August, both armies began a counter-offensive at the Battle of the Trouée de Charmes and regained the line of 14 August, by early September. Ardennes Battle of the Ardennes, 21–23 August Joffre issued instructions on 18 August but held back the Third and Fourth armies because air and cavalry reconnaissance found few German troops opposite the two armies, only a large force moving north-west away. On 19 August, the Fourth army of General Fernand de Langle de Cary, was ordered to occupy the bridges over the Semois but not to advance into Belgium until the German offensive began. A premature attack would advance into a trap, rather than give time for the Germans to empty Luxembourg of troops before the French advanced. On 20 August the German armies in the south attacked the French first and Second armies and next day the Third and Fourth armies began their offensive. The Fourth Army crossed the Semois and advanced