By the middle of May 1917 the British efforts on the Arras front had achieved the success which had been aimed at; and the offensive having been sufficiently prolonged to assist the French in their operations on the Chemin des Dames, the Commander-in-Chief was free to turn his attention to the northern area of the British lines. The first phase of the operation was opened on the 7th June, when a brilliant attack by the Second Army (Plumer) carried the British line forward over the Messines and Wytschaete Ridges, from which the Germans had dominated our positions since October 1914. This operation, which was one of the most completely successful of the whole War, resulted by the 14th June in the advancement of practically the whole Second Army front from the River Warnave to Klein Zillebeeke. One by one the points of vantage held by the enemy since the beginning of siege warfare were being wrested from his grasp. In succession the Thiepval Ridge, the Bucquoy Ridge, the Vimy Ridge and finally the Messines-Wytschaete Ridge had fallen into our hands, and there remained of this long series of heights only the series of ridges which from Zillebeeke to Passchendaele dominate Ypres on the east and north sides. It was towards these hills that the British offensive efforts were now directed. The opening day of the offensive had originally been fixed for the 25th July 1917, but owing to the intensity of our bombardment the enemy in anticipation of attack had withdrawn his guns, and the attack was therefore postponed in order that the British guns might be correspondingly advanced. Combined with the systematic bombardment of the enemy's trenches, strong points and communications, a definite air offensive which ensured our local supremacy in this respect, and also severe gas shelling, were undertaken. The front of attack extended for some fifteen miles from Deulemont on the right to Boesinghe on the Yser Canal--the main attack being entrusted to the Fifth Army (Gough) on a front of about seven miles from the Zillebeeke-Zandvoorde Road to Boesinghe. The Second Army on the right was to make only a limited advance with the chief objects of widening the front of attack and distributing the enemy's resistance. At the same time the French on the extreme left (or north) would co-operate in the marshes of the Yser.