The heroic stand made by the Belgians at Liege purchased invaluable time for the preparations of the Allies. When, on the 17th of August, the last fort of Liege fell, the great wheel of the German northern armies began to revolve. Its pace was to be regulated by the pace of the armies nearest to its circumference; that is to say, the First Army, under von Kluck, and the Second Army, under von Buelow. Three divisions of cavalry were to advance against the line Antwerp-Brussels-Charleroi, moving westward across Belgium in order to discover whether a Belgian army was still in being, whether the British had landed any troops, and whether French forces were moving up into northern Belgium. The Belgian army retired within the defensive lines of Antwerp, and by the 20th of August Brussels was in the hands of the enemy. By the 22nd, von Buelow's army had entered Charleroi and was crossing the Sambre. The repulse of the French centre in the Ardennes left the British army and the French Fifth Army completely isolated on the front Mons-Charleroi. The French Fifth Army began to retreat. On Sunday morning, the 23rd of August, von Kluck's army came into action against the British position at Mons. The British army had taken up its position in high hopes. It was not a British defeat which began the retreat from Mons, and the troops were not well pleased when they were ordered to retire. But the retreat was inevitable, and the most that the British could do was by rearguard actions to put a brake upon the speed of the advancing enemy until such time as they should be able to form up again in the Allied line and assail him. Much depended on their power to gain information concerning the movements of the enemy, so that they might know their own dangers and opportunities. Von Kluck had at first no definite news of the whereabouts of the British army. As late as the 20th of August the German Supreme Command had issued a communication to the German armies stating that 'a disembarkation of British Forces at Boulogne and their employment from the direction of Lille must be taken into account. It is the opinion here, however, that a landing on a big scale has not yet taken place.' General von Zwehl, Commander of the Seventh Reserve Corps, writing in September 1919, tells how the Germans had no reliable information concerning the British expeditionary force. 'It was only on the 22nd of August,' he says, 'that an English cavalry squadron was heard of at Casteau, six miles north-east of Mons, and an aeroplane of the English fifth flying squadron was shot down that had gone up from Maubeuge. The presence of the English in front was thus established, although nothing as regards their strength.' The first news that reached General von Kluck of the presence of the British forces came to him from a British, not from a German, aeroplane. [Illustration: Map Illustrating Aerial Reconnaissance Area. 19th to 24th Aug. 1914]