A preliminary obstacle to be overcome was the Messines-Wytschaete Ridge which dominated Ypres and the whole of the line from which an offensive in Flanders could start. Preparations to deal with it had been in progress since early in the year, and heavy guns had also been mounted on our positions near Nieuport. The plan indeed had been in Haig's mind since November 1916, and even earlier than that Sir Herbert Plumer had been training the Second Army for its task; it had had no serious fighting since the second battle of Ypres in April 1916, the battle of the Somme having been fought by the Fourth and Fifth, and that of Arras by the First and Third. The victory, however, was to be largely a triumph of engineering science. For nearly a year and a half tunnelling had been in progress under the ridge, and at dawn on 7 June nineteen huge mines were exploded beneath the enemy's lines in the greatest artificial eruption that had ever shattered the earth's crust. Ten days' surface bombardment had already obliterated much of the German defences, and it says something for the German moral that any resistance was offered at all when our troops advanced over the ruins of the soil. Messines was cleared by New Zealanders by 7 a.m., Wytschaete fell by noon before Ulstermen and Irish Nationalists fighting side by side, and Welshmen captured Oosttaverne a few hours later. The battle could not have have been better staged to exhibit the co-operation of the British Empire and of mechanical science and human valour. A few days later Australians pushed on to Gapaard and La Potterie in the direction of Warneton, and the Germans withdrew from all their positions in the salient. The danger to Ypres which had threatened for over two years and a half and had cost so much in British blood, had at last been exorcised, and from being an almost forlorn hope of defence the Ypres salient became the base of a promising advance (see Map, p. 288).