Yet there was a great deal more than that involved. It would be gravely unjust to German strategy to suppose that they had not considered the possible results of a failure in their plan of attack. The German General Staff was fully prepared with its defensive line in case Paris did not fall. The sapping and mining corps, the engineer corps, did not join in the advance on the Marne. For a week they had been working with indomitable energy on the Aisne to prepare what proved to be an invulnerable natural fortress, strong as the Rock of Gibraltar. Months before the war began, Germany had not only laid out a basis of battle on a favorable terrain, but she had also laid out in detail the manner in which a defensive position was to be taken up, should this prove necessary. She knew that if she failed at Paris, the loss of life would have been fearful. The German system of fighting in massed formations ensured that. It would, therefore, be all the more necessary that the defense should be made with machinery. If the heights were to be taken, let flesh and blood do it. The Germans had been slaughtered in attacking Liége. Let the Allies be slaughtered in attacking the Aisne. Every foot of land had been mapped and studied, the heaviest artillery in the world was available, and the ammunition supply system was in full operation. Let them come! Germany had prepared a marvelous attack which was within an ace of success and was prevented from the accomplishment of its final and full aim only by three things, each, in its way, a glory to one of the Allied Nations: the valor of the defense of the Belgians at Liége; the dogged courage of the British in the fighting retreat from Mons; and the superb dash of the French when they shattered the German line at the Marne. All three were needed to save France. The battle of the Aisne consisted simply of the efforts of the English and French to gain those forbidding and strongly protected heights. Von Kluck, given all the men and artillery he needed, drove back Manoury in the space of a few hours. The British crossed by a superb frontal attack, which ranks as one of the bravest deeds in modern warfare, and were wiped out. D'Esperey crossed the Aisne east of Bourg, only to find that the Craonne plateau was unassailable. By Friday, September 18, Joffre was compelled to realize that the bluffs above the Aisne had been turned into an impregnable open-air fortress, not to be stormed by flesh and blood. For Germany one thing was lacking, a strong Line of Communication. The main railroad to Coblentz, with a branch to Metz, passed through Rheims. If Germany were to have the vast supplies she needed, she must take Rheims or content herself with the weeks of delay which the Belgian route required. Rheims was imperative. But Foch held Rheims!