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.
As planned, the main framework of the defense in depth was based upon machine-gun positions, protected by belts of wire entanglements so placed, in relation to the field of fire of the machine guns, that they were enfiladed over their entire length. The whole area was compartmented in such a way that the loss of ground at any one point could be localized, and would not cause a forced retirement from adjoining areas.
Machine-gun emplacements of the Champagne type were constructed, and dugout accommodation for the machine-gun detachments was provided in the deep tunnels of these emplacements.
This framework was completed as rapidly as possible by trenches and by defended localities organized for all-round defense.
A great many dugouts were made to accommodate the garrisons of these localities, and for dressing stations and battle headquarters. Advantage was taken of the possibility of utilizing the subways tunneled in 1916-17 for the attack on Vimy Ridge, and in addition steps were taken to create an obstacle on the southern flank of Vimy Ridge by the construction of dams to enable the valley of the Scarpe to be flooded as required. Trial inundations were made to insure the smooth working of these arrangements.
A great deal of care was given to the distribution of the artillery in relation to the policy of defense. Three systems of battery positions were built so as to distribute the guns in depth and sited so as to cover the ground to the northeast, east, and south, in case the flanks of the corps should be turned. These batteries were protected with barbed-wire entanglements and machine-gun positions against a sudden penetration of the enemy, and they were designed to become the natural rallying points of infantry in this eventuality.
Successive lines of retirement were also prepared, battery positions were selected, organized, and marked, cross-country tracks were opened up, and observation posts, echeloned in depth, were located and wired in.