If instead of two tanks we use four, a much more effective operation may be carried out. The tanks can either penetrate at one place, and wheel outwards by pairs (see diagram 8), or by pairs penetrate at two separate points and wheel inwards, pinching on the centre (see diagram 9), or two can wheel to a flank and two proceed straight ahead (see diagram 10) and threaten the enemy’s line of retreat. When this latter operation is contemplated it is as well to make use of at least six machines, better twelve, _i.e._ a complete company of tanks. If six machines are used they normally should strike the enemy’s line at approximately the same place; from there one half section should go straight forward and one to each flank, forming what has been called the “Trident formation” (see diagram 11). If twelve machines are employed, then each section of four tanks strikes the trench at a separate point, the centre section forging straight ahead and the flanking sections moving inwards and outwards as depicted in diagram 12.
Particular attention should be paid to the outward movements of the flanks, for, as the flanks of our own penetrating or attacking force are generally the most vulnerable points, if we can push forward offensive wings on these flanks we shall not only be protecting our own flanks from attack, by giving the enemy no time to attack in, but we shall be protecting our central line of advance as well. The force operating along this central line not only depends for its movement forward on the security of its flanks, but also on the size of the base of operations; the broader this base the more secure will it be, for the one thing an attacking army wishes to avoid is getting into a pocket on the interior of which all hostile fire is concentrated.
From the above elementary movements can be worked out a whole series of battle formations according to the various arms which are to be employed. The following three were those generally used by tanks from the battle of Cambrai onwards:
(1) _An attack against trenches with an artillery barrage_ (see diagram 13).--Three tanks in line at 100 to 200 yards’ interval, followed by infantry in sections, each section forming an independent fighting unit advancing in single file and attacking in line, the whole forming one firing line. Behind this firing line should advance one tank and a certain number of infantry sections as a support. Reserves can be added as necessary.