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 towards Neufchâteau and the Third Army of General Pierre Ruffey attacked towards Arlon, as a right flank guard for the Fourth army.