To create a complete gap, it is necessary:
(a) To take the first line of the hostile defense (zone of the first trenches and centers of resistance);
(b) To take the second line of defense;
(c) To prevent the enemy from reestablishing a barrier by the aid of reinforcements brought up in haste beyond the zone already fortified.
To overcome successively these difficulties, one must have:
(a) A first line of attack composed of several waves of assault with (as an element of preparation) a formidable artillery (field, heavy, and torpedo guns) minutely regulated.
(b) A second line of attack as strong as the first, except perhaps in front of the centers of resistance, sent straight to the front all in one piece exactly like the first line. The same precise and effective artillery preparation is not here present, but it is compensated for by groups of light guns and machine guns destined rapidly to destroy all resistance. Accompanying batteries (batteries d’accompagnement) start as soon as the first trenches are taken.
(c) A reserve without initial assignment, destined to reinforce any point and conquer any irreducible or hindering resistance. This reserve is entirely at the disposition of the superior commander, while the first and second lines are no longer in his hands after they are in their parallel of departure. On the 9th of May, this reserve was made up of the troops which should have normally composed the second line of attack, which did not exist. This explains the disastrous delay of its engagement, which was furthermore very hesitating, because it tried to maneuver before having broken through and waited for the mêlée to clear away in order to maneuver.
(d) Cavalry, auto-cannon, auto-machine guns, battalions of infantry on automobiles with pioneer crews to clear the roads.—Large units, ready to commence new combats, capable of being brought up within two or three hours.—Do not, after the hole has been pierced, depend any longer upon the regiments who made it.