Anticipating some such attack, Haig had deemed it wise to relieve the two Portuguese divisions which held part of the front between the Lys and La Bassée of their arduous responsibility; but he could only replace them by weary British divisions, and the change had only been half effected when, on 9 April, Ludendorff s attack began after the usual bombardment with gas and high-explosive on the 8th. The Portuguese broke fairly soon, the British flanks on either side were turned, and the whole centre had gone in a few hours. By night the Germans had captured Fleurbaix, Laventie, Neuve Chapelle, Richebourg, and Lacouture, and were on the Lys from Bac St. Maur almost as far as Lestrem. But the key-position at Givenchy was splendidly held by the 55th Division, which set a permanent limit to the German success and prevented it from obtaining anything like the dimensions of the March offensive. It continued, however, to develop on the north. On the 10th Bois Grenier fell, Armentières was evacuated, and the Germans poured across the Lys, taking Estaires, Steenwerck, and Ploegstreet and threatening the Messines ridge. That, too, followed on the 11th, while farther south the Germans secured Neuf Berquin and Lestrem. On the 12th they got into Merris and Merville and advanced to the La Bassée canal, threatening to cross it and outflank Béthune on the north-west. Here, however, they were held up in front of Robecq, between the canal and the forest of Nieppe, and turned to exploit their advantage farther north. Their advance here was slower, but by the 16th they had mastered Wytschaete, Wulverghem, Neuve Eglise, Bailleul, and Meteren, and were facing the line of hills running from Mt. Kemmel to the Mt. des Cats.
British and French reinforcements were now arriving in considerable numbers, and Ludendorff would have been prudent to rest on his laurels. He had made a pronounced bulge in our line, had diverted forces from other sectors of the defence, and compelled us to evacuate our dearly-purchased gains of the Flanders campaign in the preceding autumn. On the other hand, he had lengthened instead of shortening his own line, he had achieved no strategical object, and his troops were left in a salient which invited attack. Unless he could win the heights from Mt. Kemmel to Mt. des Cats, which commanded the country to the coast, he would be in a worse situation for defence than he was before. He was thus driven to prolong the effort, pour fresh divisions into the battle, and convert a diversion into a major operation. Doubtless popular visions of the Channel ports and the bombardment of London reinforced the sounder military reasons for persistence. There were three obvious lines of attack--on the Belgian front north of Ypres, on the Kemmel range, now held partly by French troops, and on Béthune. The first was defeated on the 17th by a brilliant Belgian resistance, and the third was repulsed on the 18th before Hinges and at Givenchy; but the second was longer delayed and more stubbornly pushed.