On War

by Carl von Clausewitz

4. As tactical points d’appui. As the diameter of the zone covered by the fire of even a very inferior class of fortifications is usually some leagues, fortresses may be considered always as the best points d’appui for the flanks of a position. A lake of several miles long is certainly an excellent support for the wing of an army, and yet a fortress of moderate size is better. The flank does not require to rest close upon it, as the assailant, for the sake of his retreat, would not throw himself between our flank and that obstacle.

5. As a station (or stage). If fortresses are on the line of communication of the defensive, as is generally the case, they serve as halting places for all that passes up and down these lines. The chief danger to lines of communication is from irregular bands, whose action is always of the nature of a shock. If a valuable convoy, on the approach of such a comet, can reach a fortress by hastening the march or quickly turning, it is saved, and may wait there till the danger is past. Further, all troops marching to or from the army, after halting here for a a few days, are better able to hasten the remainder of the march, and a halting day is just the time of greatest danger. In this way a fortress situated half way on a line of communication of 30 miles shortens the line in a manner one half.

6. As places of refuge for weak or defeated corps. Under the guns of a moderate sized fortress every corps is safe from the enemy’s blows, even if no entrenched camp is specially prepared for them. No doubt such a corps must give up its further retreat if it waits too long; but this is no great sacrifice in cases where a further retreat would only end in complete destruction.

In many cases a fortress can ensure a few days’ halt without the retreat being altogether stopped. For the slightly wounded and fugitives who precede a beaten army, it is especially suited as a place of refuge, where they can wait to rejoin their corps.

If Magdeburg had lain on the direct line of the Prussian retreat in 1806, and if that line had not been already lost at Auerstadt, the army could easily have halted for three or four days near that great fortress, and rallied and reorganised itself. But even as it was it served as a rallying point for the remains of Hohenlohe’s corps, which there first resumed the appearance of an army.

It is only by actual experience in war itself that the beneficial influence of fortresses close at hand in disastrous times can be rightly understood. They contain powder and arms, forage and bread, give covering to the sick, security to the sound, and recovery of sense to the panic-stricken. They are like an hostelry in the desert.

In the four last named purposes it is evident that the active agency of fortresses is called more into requisition.

7. As a real shield against the enemy’s aggression. Fortresses which the defender leaves in his front break the stream of the enemy’s attack like blocks of ice. The enemy must at least invest them, and requires for that, if the garrisons are brave and enterprising, perhaps double their strength. But, besides, these garrisons may and do mostly consist in part of troops, who, although competent to duty in a garrison, are not fit for the field—half trained militia, invalids, convalescents, armed citizens, landsturm, etc. The enemy, therefore, in such case is perhaps weakened four times more than we are.

This disproportionate weakening of the enemy’s power is the first and most important but not the only advantage which a besieged fortress affords by its resistance. From the moment that the enemy crosses our line of fortresses, all his movements become much more constrained; he is limited in his lines of retreat, and must constantly attend to the direct covering of the sieges which he undertakes.

Here, therefore, fortresses co-operate with the defensive act in a most extensive and decisive manner, and of all the objects that they can have, this may be regarded as the most important.