Defense of Swamps
Very large wide swamps, such as the Bourtang Moor in North Germany, are so uncommon that it is not worth while to lose time over them; but we must not forget that certain lowlands and marshy banks of small rivers are more common, and form very considerable obstacles of ground which may be, and often have been, used for defensive purposes.
Measures for their defense are certainly very like those for the defense of rivers, at the same time there are some peculiarities to be specially noticed. The first and principal one is, that a marsh which except on the causeway is impracticable for infantry is much more difficult to cross than any river; for, in the first place, a causeway is not so soon built as a bridge; secondly, there are no means at hand by which the troops to cover the construction of the dyke or causeway can be sent across. No one would begin to build a bridge without using some of the boats to send over an advanced guard in the first instance; but in the case of a morass no similar assistance can be employed; the easiest way to make a crossing for infantry over a morass is by means of planks, but when the morass is of some width, this is a much more tedious process than the crossing of the first boats on a river. If now, besides, there is in the middle of the morass a river which cannot be passed without a bridge, the crossing of the first detachment of troops becomes a still more difficult affair, for although single passengers may get across on boards, the heavy material required for bridge building cannot be so transported. This difficulty on many occasions may be insurmountable.
A second peculiarity of a swamp is, that the means used to cross cannot be completely removed like those, used for passing a river; bridges may be broken, or so completely destroyed that they can never be used again; the most that can be done with dykes is to cut them, which is not doing much. If there is a river in the middle, the bridge can of course be taken away, but the whole passage will not by that means be destroyed in the same degree as that of a large river by the destruction of a bridge. The natural consequence is that dykes which exist must always be occupied in force and strenuously defended if we desire to derive any general advantage from the morass.
On the one hand, therefore, we are compelled to adopt a local defense, and on the other, such a defense is favored by the difficulty of passing at other parts. From these two peculiarities the result is, that the defense of a swamp must be more local and passive than that of a river.
It follows from this that we must be stronger in a relative degree than in the direct defense of a river, consequently that the line of defense must not be of great length, especially in cultivated countries, where the number of passages, even under the most favorable circumstances for defense, is still very great.
In this respect, therefore, swamps are inferior to great rivers, and this is a point of great importance, for all local defense is illusory and dangerous to an extreme. But if we reflect that such swamps and low grounds generally have a breadth with which that of the largest rivers in Europe bears no comparison, and that consequently a post stationed for the defense of a passage is never in danger of being overpowered by the fire from the other side, that the effects of its own fire over a long narrow dyke is greatly increased, and that the time required to pass such a defile, perhaps a quarter or half a mile long, is much longer than would suffice to pass an ordinary bridge: if we consider all this, we must admit that such low lands and morasses, if means of crossing are not too numerous, belong to the strongest lines of defense which can be formed.
An indirect defense, such as we made ourselves acquainted with in the case of streams and rivers, in which obstacles of ground are made use of to bring on a great battle under advantageous circumstances, is generally quite as applicable to morasses.
The third method of a river-defense by means of a position on the enemy’s side would be too hazardous on account of the toilsome nature of the crossing.
It is extremely dangerous to venture on the defense of such morasses, soft meadows, bogs, etc., as are not quite impassable beyond the dykes. One single line of crossing discovered by the enemy is sufficient to pierce the whole line of defense which, in case of a serious resistance, is always attended with great loss to the defender.