The French instruction for the siege warfare
Attack. According with the French Instruction générale sur la guerre de siège, published on 30 July
The advancing troops surrounded the fortress at a proper distance, attacking and repelling the advanced enemy units, and stormed their outposts (primary line of resistance). The investment line should be strongly fortified and placed as closer as possible to the enemy forts, but in such a way to keep it from the effective fire of the powerful artillery of the main defence line.
The commander of the siege army fixed the plan of the attack, according with result of the reconnaissance and the suggestions of the heads of the artillery and engineer parks. Then in the area of the attack the infantry took possession of its positions to protect the artillery. This action would cause violent fighting, and the attacking troops should be supported by the fire of the heavy field artillery and, if necessary, of some siege batteries.
When the troops were deployed along the line of attack, the artillery, thanks to a great superiority of guns and a large amount of shells, tried to silence the enemy artillery. Only after this goal had been achieved, the artillery could open the fire against the obstacles that hindered the advance of the infantry. It fired to parapets, redoubts, communications, flanking works and shelled the counterscarps of the ditches, to open some breaches in it. If necessary, part of the batteries should be moved forward to occupy closer emplacements.
As soon as the artillery and the first lines of the advancing infantry have obtained the fire superiority, the commander of the siege army established new positions for the troops of the different sectors. The advance was carried out in the open or through underground passages. Finally the commander fixed the moment for the final attack to the fortress.
If part of the main line of forts fell, the second line of defence should be quickly attacked. If this was not possible, the attack should go on according with the rules shown above, taking anyway advantage from the enemy demoralization. The fall of the fortress could be speed up by means of a heavy bombing.
Defence. The defence of a strongpoint began on the outside of the fortress itself and should have a marked offensive nature. The defenders, if forced by the enemy, should withdraw only step by step. The fortress as a whole was composed by many different lines of fortifications:
the advanced line of defence,
2. the main defence line, which included the forts girdle;
3. the second line of defence, usually formed by old works;
4. the core of the fortress (noyau);
5. the citadel (if existed).
The artillery was composed by security (permanent) batteries, mobilization batteries, reserve batteries and mobile batteries (armed with teamed field and heavy artillery pieces).
The main reserve of the fortress offered the first resistance against the enemy attack beyond the primary line of resistance. If repelled, it withdrew to the primary line of resistance, strengthened by field and semi-permanent works. As soon as the direction of the enemy attack was clear, the line of defence was reinforced with the reserve and the troops of the quiet sectors. The reserve, waiting in covered shelters, should be ready to go into the action in order to support the first line if the enemy appeared to prevail. At the same time the second line of defence was prepared.
The whole artillery fired against the attacking batteries, to keep them to get the upper hand. But if this happened, all the supporting batteries one after another were withdrawn to the second line of defence, while the main defence line was defended only by the armoured batteries. The field artillery stood in reserve to repel the attack. The progress of the siege works were hindered by the sudden fire of the field guns and by sorties. The troops retreated step by step, blocking the breaches with hindrances and storming the attackers with handgrenades.
The defence of the second line of defence should go on according with the rules shown above, with the support of the heavy guns placed in the works and the flanking batteries and of the howitzers and mortars deployed in covered emplacements behind. When the enemy fire began again to get the upper hand, the artillery still able to fight were moved to the core of the fortress or to the works not menaced by the enemy. The troops should hold out into the core, the citadel and even into the works not attacked by the enemy. But if the fortress was forced to surrender, the governor of the place should share the fate of the rest of the garrison.