The West Yorkshire troops did not come in till close to the end of this middle battle, and we shall presently be more fully concerned with the ‘other projects’ elsewhere. But we can imagine what it meant to those spent and battle-weary soldiers to ‘maintain pressure’ beyond the hope of progress. ‘Physical exhaustion,’ we read, ‘placed narrow limits on the depth to which each advance could be pushed’; and how far those limits should be forced was a matter of very difficult discretion. ‘Time after time,’ runs the Despatch, ‘the practically beaten enemy was enabled to re-organize and relieve his men, and to bring up reinforcements behind the sea of mud which constituted his main protection’; and at what point a ‘practically beaten’ enemy should be left behind his barrier of mud was, again, very hard to decide. Hard and difficult decisions for the High Command; but the hardship and the difficulty of the fighting fell heavily on the fighting men, and the Summer-battle of 1917, which was prolonged far beyond the Summer, entailed, as Sir Douglas Haig tells us, ‘almost superhuman exertions on the part of the troops of all arms and services.[85]’ The great Commander chose his word well. If the triple battle of 1917 were to be fought out again, with all the conditions constant except those which strategists could vary, there would be, conceivably, a new time-table and a new distribution of effort at Arras, Passchendaele and Cambrai: there would still be the ‘superhuman’ effort to overcome the German advantage of irregular, murderous blockhouses, like Martello-towers sunk in a sea of mud, and of not less irregular rain. We come to closer quarters with this middle battle. It opened on June 7th with an explosion of nineteen mines, which caused enormous rents in the enemy front-line trenches, and which effectively assisted the Artillery and the Air Force in their preparations for the Infantry advance. Impressive from a spectacular point of view, it was no sudden thing, this explosion. It represented many months of patient labour by highly-skilled miners and engineers, the memory of whose devotion to duty, under conditions of constant horror, should help, in industrial times, to soften acerbities at home. It was, further, the great surprise of the attack. British enterprise had to burrow underground in order to escape the observation of an enemy, who, since 1915, when the Ypres salient was inevitably contracted,[86] had occupied all the commanding ground in a stretch of country where 60 feet was the measure of a mountain. Messines, Wytschaete and Oostaverne were all captured on that first day (June 7th), together with more than 7,000 prisoners and 450 pieces of Artillery. General Sir Herbert Plumer and the Second Army, who had acted as wardens of these marches through so many weary and exacting months, reaped a swift reward in the second week of June.