On War

by Carl von Clausewitz

aa. When the rear is perfectly safe (Torres Vedras).

bb. When we foresee that the enemy’s force is not strong enough formally to invest us in our camp. Should the enemy attempt the investment with insufficient means, then we should be able to sally out of the camp and beat him in detail.

cc. When we can count upon relief like the Saxons at Pirna, 1756, and as took place in the main at Prague, because Prague could only be regarded as an entrenched camp in which Prince Charles would not have allowed himself to be shut up if he had not known that the Moravian army could liberate him.

One of these three conditions is therefore absolutely necessary to justify the choice of a strong position for the main body of an army; at the same time we must add that the two last are bordering on a great danger for the defensive.

But if it is a question of exposing an inferior corps to the risk of being sacrificed for the benefit of the whole, then these conditions disappear, and the only point to decide is whether by such a sacrifice a greater evil may be avoided. This will seldom happen; at the same time it is certainly not inconceivable. The entrenched camp at Pirna prevented Frederick the Great from attacking Bohemia, as he would have done, in the year 1756. The Austrians were at that time so little prepared, that the loss of that kingdom appears beyond doubt; and perhaps, a greater loss of men would have been connected with it than the 17,000 allied troops who capitulated in the Pirna camp.

c. If none of those possibilities specified under a and b are in favour of the aggressor; if, therefore, the conditions which we have there laid down for the defensive are fulfilled, then there remains certainly nothing to be done by the assailant but to fix himself before the position, like a setter before a covey of birds, to spread himself, perhaps, as much as possible by detachments over the country, and contenting himself with these small and indecisive advantages to leave the real decision as to the possession of territory to the future. In this case the position has fulfilled its object.

3. Entrenched camps near fortresses.—They belong, as already said, to the class of entrenched positions generally, in so far, as they have for their object to cover not a tract of territory, but an armed force against a hostile attack, and only differ in reality from the other in this, that with the fortress they make up an inseparable whole, by which they naturally acquire much greater strength.

But there follows further from the above the undermentioned special points.

a. That they may also have the particular object of rendering the siege of the fortress either impossible or extremely difficult. This object may be worth a great sacrifice of troops if the place is a port which cannot be blockaded, but in any other case we have to take care lest the place is one which may be reduced by hunger so soon that the sacrifice of any considerable number of troops is not justifiable.

b. Entrenched camps can be formed near fortresses for smaller bodies of troops than those in the open field. Four or five thousand men may be invincible under the walls of a fortress, when, on the contrary, in the strongest camp in the world, formed in the open field, they would be lost.