One of the simplest methods of sealing off a breakthrough or eliminating a penetration is the frontal counterattack. Usually, such a counterattack can be launched only if the breakthrough is minor and can be localized, and if both shoulders are secure. Moreover, sufficient reserves must be available to close the breach by a quick counterthrust before the enemy is able to widen the gap. Once hostile preparations for a breakthrough have been clearly recognized, it is most effective to move the reserves close to the rear of the threatened sector. While the reserves must be close enough for instantaneous effective employment, they should be sufficiently removed from the front line so as not to forfeit prematurely their freedom of maneuver. In their assembly areas the reserves must be concealed from enemy observation and air attacks and must not be exposed to hostile preparation fire. Obviously, reserves should have maximum fire power and mobility; armored divisions come closest to these requirements because they combine tremendous striking force with concentrated fire power. Infantry supported by assault guns will often restore the situation so long as the breakthrough is local.
A counterattack is far more complicated if, before its effect is felt by the enemy, the shoulders begin to crumble, the breach is widened, and the enemy attack gains ground in depth. But even in this event, it is best to maintain the tactical integrity of the reserve so that upon commitment it can overrun the enemy infantry in one powerful thrust and regain the key positions of the former line. Only then should attempts be made to close the smaller gaps by flanking actions. As a countermeasure against the disintegration of the shoulders and as support to the flanking actions, it will prove effective to protect the open flanks of the break-through area with artillery and to assemble small local reserves behind them. Frequently one infantry company supported by assault guns will suffice for this purpose.
Whenever the enemy achieves a major breakthrough that causes the collapse of a wide sector of the front (thirty miles or more), the local reserves will always be insufficient to close the gap by frontal counterattack. Piecemeal commitment of individual divisions in a gap of this width will simply lead to their engulfment by the advancing hostile avalanche. Only a strong force consisting of several corps will be able to stem the tide and halt the enemy advance in the depth of the defense or to close the gap by a counterattack. There will usually be a considerable time lapse, however, before a force of such strength can be released from other sectors and moved to the breakthrough area. Meanwhile, attempts must be made to narrow the breach by withdrawing to a shorter line and by strengthening the resistance in the sectors adjacent to the gap.