All day long on April 11, 1918, the Germans were hurling great masses of troops against the British lines on the northern front, from La Bassée to the Ypres-Commines Canal southeast of Ypres. As the result of these heavy assaults Haig's troops were forced back north of Estaires and Steenwerck. The evacuation of Armentières followed. The German advance on the previous day between Warneton and the Lys on the north and their drive to the south of Armentières had subjected the place to attack from three sides. For the British to attempt holding the town under these conditions would have caused a useless sacrifice of troops. Armentières had no military importance, but the Messines-Wytschaete Ridge and the Passchendaele Ridge to the north of it were of great value. As long as these positions held out the British lines were safe. The Germans were confronted with the same situation that prevailed on the Somme front. The German push toward Amiens had been held up by the British possession of the Heights of Vimy and of the Notre-Dame-de-Lorette. These formed the hinge on which the whole British line swung as it moved westward. The same thing was happening in the north. The British center had given way under the fury of the German attacks, but the flanks held fast. On the north the Messines Ridge blocked the way of the German advance. For in case of a westward push both German flanks would be in great danger from this bastion which commanded the entire field of maneuver. Their front and rear as well as flanks would be subjected to destructive artillery fire from this ridge which therefore became the center of the fighting in the north. The Germans recognizing the supreme importance of the ridge made determined and valiant efforts to capture the position. They launched one attack after another that were costly and brought no results. They entered the British lines in the village of Messines, but were promptly driven out in a brilliant counterattack and the important positions remained in British hands. In the area west and northwest of Armentières the British troops were forced back in the fighting that continued throughout the day on April 12, 1918. The British retired to the neighborhood of the railway south of Bailleul, where the heaviest fighting continued without respite. The Germans captured Merville and drove the British forces operating near Ploegsteert to retire to new positions in the neighborhood of Neuve Église.