Haig was compelled to protract the fighting in the Arras area so long as the French on the Aisne required his aid; but by the end of May he was free to turn his attention to the plan which, as early as the previous November, had been his main preoccupation. This was an offensive against the enemy in Flanders, with the aim of clearing the Belgian coast and turning the northern flank of the whole German defence system in the West. It was a scheme which, if successful, promised the most far-reaching results. It would destroy the worst of the submarine bases; it would restore to Belgium her lost territory, and thereby deprive Germany of one of her most cherished bargaining assets; it would cripple the enemy communications with the depôts of the Lower Rhineland. But time was the essence of the business. The blow must be struck at the earliest possible hour, for each week’s delay meant the aggrandisement of the enemy. [Sidenote: _June 7._] Haig’s first business was to clear his flanks for the coming attack, and on the 7th June, by one of the most perfect operations in the campaign, he won the Messines-Wytschaete ridge at a single bound. His next step was the advance east of Ypres. The famous Salient had during three years been gradually contracted till the enemy front was now less than two miles from the town. The Germans held all the half-moon of little hills to the east, which meant that any preparations for attack would be conducted under their watchful eyes. They were very conscious of the importance of the position, and the wary general who now commanded their IV. Army was not likely to be taken by surprise. This was Sixt von Armin, who had commanded the 4th Corps at the Somme, and had there shown himself one of the most original and fruitful tacticians on the enemy’s side. The Battle of Messines was over by the 12th June, but for various reasons it was not till late in July that the date of the main advance could be fixed. It was now more than ever a race against time, for the precarious weather of autumn was approaching; and, unless the advance proceeded strictly according to time-table, it ran a grave risk of failure. The high ground east of the Salient must be won in a fortnight to enable us to move against the enemy bases in West Flanders and clear the coast-line. The nature of the countryside made any offensive a gamble with the weather, for the Salient was, after Verdun, the most tortured of the Western battlefields. Constant shelling of the low ground west of the ridges had blocked or diverted the streams and the natural drainage, and turned it into a sodden wilderness. Weather such as had been experienced on the Somme would make of it a morass where tanks could not be used, and transport could scarcely move, and troops would be exposed to the last degree of misery.