The Third Army plan of operations was as follows:
(i) To break the German defensive system between the canal St. Quentin and the canal Du Nord.
(ii) To seize Cambrai, Bourlon wood, and the passages over the river Sensée.
(iii) To cut off the Germans in the area south of the Sensée and west of the canal Du Nord.
(iv) To exploit the success towards Valenciennes.
This operation, for its initial success, depended on the penetration of all lines of defences, including the Masnières-Beaurevoir line, which in its turn depended on the seizing of the bridges at Masnières and Marcoing.
The force allotted for this attack was—two corps of three infantry divisions each; the Tank Corps of nine battalions—378 fighting tanks and 98 administrative machines; a cavalry corps, and 1,000 guns.
The attack was to be carried out in three phases. In the first the infantry were to occupy the line Crèvecœur, Masnières, Marcoing, Flesquières, canal Du Nord; the leading cavalry division was then to push through at Masnières and Marcoing, capture Cambrai, Paillencourt, and Pailluel (crossing over the river Sensée), and move with its right on Valenciennes; whilst this was in progress the IIIrd Corps, which formed the right wing of the Third Army, was to form a defensive flank on the line Crèvecœur, La Belle Etoile, Iwuy; the cavalry were then to cut the Valenciennes-Douai line and so facilitate the advance of the IIIrd Corps in a north-easterly direction. The second and third phases were to be carried out by the IVth Corps, which formed the left wing of the Third Army, and were to consist firstly in opening the Bapaume-Cambrai road and occupying Bourlon and Inchy, and secondly, in opening the Arras-Cambrai road and advancing on the Sensée canal and so to cut off the German forces west of the canal Du Nord.
The ground to be fought over consisted chiefly in open, rolling downland, very lightly shelled, and consequently most suitable to tank movement. The main tactical features were the two canals which practically prohibited the formation of tank offensive flanks and so strategically were a distinct disadvantage to what was meant to be a decisive battle. Between these two canals were two important features—the Flesquières-Havrincourt ridge and Bourlon hill. A third very important feature, known as the Rumilly-Seranvillers ridge, ran parallel to and north of the St. Quentin canal between Crèvecœur and Marcoing; without the occupation of this ridge a direct attack from the south on Bourlon hill could only take place under the greatest disadvantage.
The German defences consisted of three main lines of resistance and an outpost line: these lines were the Hindenburg Line, the Hindenburg Support Line, and the Beaurevoir-Masnières-Bourlon line, the last being very incomplete. The trenches for the most part were sited on the reverse slopes of the main ridges, and consequently direct artillery observation on them from the British area was impossible. They were protected by immensely thick bands and fields of wire arranged in salients so as to render their destruction most difficult. To have cut these bands by artillery fire would have required several weeks bombardment and scores of thousands of tons of ammunition.