This front was divided into five sections: Hill 70, St.-Emile, Lens, Avion, and Mericourt. The corps now settled down to the routine of trench warfare. Lieutenant General Sir A. W. Currie, of course, was in command. His dispositions provided that three of the divisions held the line while one was resting and training in reserve. Each of the divisions had approximately one month out of the line. This arrangement allowed the divisions to absorb more quickly the fresh drafts and to train rapidly the new officers and N. C. O's. The Canadians were no strangers to this sector. Having wrested it from the enemy in April, 1917, in the Battle of Vimy and subsequent actions, they had held it practically ever since, except for the short interval late in 1917 when they fought the Battle of Passchendaele. It had been considerably improved by comprehensive defenses and complete systems of trench railways, roads, and water supply were in operation. The great importance of this particular sector arose from the fact that behind Vimy Ridge lay the northern collieries of France and certain tactical features which covered the British lateral communication. "Here," as the British Commander in Chief said in one of his reports, "little or no ground could be given up." A comparatively shallow advance beyond the Vimy Ridge would have stopped the operation of the collieries, paralyzing the production of war material in France, as well as inflicting very severe hardships on the already sorely tried population. In conjunction with the shortage of shipping, which practically forbade an increase in the importation of coal from England, the loss of the northern collieries might have definitely crippled France. On the other hand, a deep penetration at that point, by bringing the Amiens-Bethune railway and main road under fire, would have placed the British army in a critical position, by threatening to cut it in two and by depriving it of vital lateral communication. The tactical and strategical results to be gained by a moderate success at that point were so far-reaching in effect that, notwithstanding the natural difficulties confronting an attack on that sector, it was fully expected that the German offensive would be directed against this the central part of the British front. With the prospect of a German offensive now confronting the Canadians, it was decided that the defenses should be revised, to take advantage of the lessons recently learned and to embody the latest methods. Moreover, instructions had been issued by the First Army defining the policy of defense to be adopted and the methods to be followed. The completion of the revised corps defenses and the execution of the new army program resulted in the organization of a very deep defended area, consisting of successive defensive systems, roughly parallel to the general line of the front and linked together by switch lines sited to protect both flanks.