By 6th April 1918 the great German thrust towards Amiens had failed, and for the moment the gate of the Somme was closed. The city was under fire, the enemy was before its gates, but his strength was exhausted and he could not advance. Therefore his chief plan—of separating the French and the British—had come to nought. Brought to a standstill, he cast about for a diversion, for he could not permit the battle to decline into a stalemate, since he was fighting against time. His main purpose remained the same, but he sought to achieve it by a new method. He would attack the British elsewhere, on some part of the front where they were notoriously weak, and compel Foch to use up his reserves in its defence. Then, when the Allied resources had shrunk, he would strike again at the weakened door of Amiens. On the German side the operation was meant to be merely subsidiary, designed to prepare the way for the accomplishment of the main task farther south. They proposed to choose a battle-ground where even a small force might obtain important results. But so stoutly did the meagre British divisions resist that the enemy was compelled to extend the battle well into May, to squander thirty-five of his fresh divisions, and to forfeit for good his chance of final victory. The new battle-ground was the area on both sides of the river Lys, between the La Bassée Canal and the Wytschaete Ridge. The German Staff knew that our front line had already been thinned to supply ten divisions for the struggle in the south, and at the moment it was weakly held, mainly by troops exhausted in the Somme battle. The enemy Staff chose their ground well. They had the great city of Lille behind them to screen the assembly. Certain key-points, such as Béthune and Hazebrouck, lay at no great distance behind the British front. The British communications were poor, while the German were all but perfect. If the enemy could break through at once between La Bassée and Armentières and capture Béthune, he could swing north-westward and take Hazebrouck and the hills beyond Bailleul, and so threaten the Channel Ports, on which the British armies depended for supplies.