From this, therefore, we deduce that the occupation of such a strong position with the main body is a measure only to be taken,—
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.