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fixed for the 25th July 1917, but owing to the intensity of our bombardment the enemy in anticipation of attack had withdrawn his guns, and the attack was therefore postponed in order that the British guns might be correspondingly advanced.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.
The offensive was finally launched on the 31st July 1917. The weather, which for a fortnight previously had been fine and dry and had seemed to predict success, broke on the day of the battle, and a merciless rain which changed the whole area of operations to a sea of mud fell without cessation for several days. The Corps in line on the 31st July were from left to right the XIV (Cavan), the XVIII (Maxse), the XIX (Watts), the II (Jacob) and the X (Morland). On the whole the day was one of marked success, the deepest advance being made in the northern sector of the attack. From Westhoek to St Julien the second German line was carried, while north of the latter village the assaulting troops passed the second line and gained the line of the Steenbeek as far as the junction with the French, whose attack had also met with complete success. South of Westhoek the enemy's resistance had been more stubborn, and his positions in Inverness Copse and Glencorse Wood which were strongly held by machine-gun posts proved an impassable obstacle. In this region, however, the German first line was carried, and our troops managed to maintain themselves far enough forward on the Westhoek Ridge to deny the enemy observation over the Ypres plain; the position gained running almost due south from Westhoek east of the line Clapham Junction—Stirling Castle—Bodmin Copse, and thence to Shrewsbury Forest, south of which the German second line was occupied as far as the Ypres-Comines Canal. South of the Canal also the Second Army achieved considerable success.
The rain, which began to fall in the afternoon, had a most disastrous effect on the British plan of attack. Movement over the shell-torn ground, which was transformed into a series of bogs, rapidly became impossible apart from a few well-defined tracks, and these naturally became marks for the enemy's guns. The labour of moving forward guns, relieving troops and completing the forward dumps and other preparations for the next bound was increased tenfold. The inevitable delay which ensued was of the greatest service to the enemy, who thereby gained a valuable respite in which he was able to bring up reinforcements.
The fighting of the next few days was, therefore, local in character and consisted in clearing up the situation and improving the British positions at various points in the line, in the course of which operations the capture of Westhoek was completed. Numerous counter-attacks by the enemy were successfully resisted, and the line gained on the 31st July was substantially held.
The 56th Division moved from the Third Army area on the 24th July to the St Omer area. The 1/4th Londons entrained at Petit Houvin for St Omer and marched to billets at Houlle, in the Eperlecques area, some five miles north-west of St Omer. The Division was now attached to the V Corps. Its training was continued during the opening phase of the battle, after which the Division moved on the 6th August to the II Corps area, the Battalion occupying billets at Steenvoorde, where the routine was resumed. On the 8th Major-Gen. D. Smith, C.B. (who had commanded the Division since 24th July, when Gen. Hull fell sick), left to command the 20th Division and, two days later, command was assumed by Major-Gen. F. A. Dudgeon, C.B.