Bet you didn’t realize the need to accessorize your fort? Yes, after digging all those ditches, piling dirt for the parapet, then addressing revetments and such, there was still work to do. This work was to supplement the defensive character of the fortification by way of obstacles or other impediments in the path of the attacker. Mahan called these “accessories, or secondary means of defense.”
The means employed as accessory usually consist of artificial obstacles, so arranged as to detain the enemy in a position where he will be greatly cut up by the fire of the work.
This is a great “sound byte” that holds true for military application even today – slow down the enemy and make him pay for showing his face in front of your works!
From this premise, Mahan discussed the nature and need for accessories:
Anything may be regarded as an obstacle to the enemy by which his attention is diverted from the assailed to his own situation; but no obstacle will be of much service to the assailed which is not within good striking distance of his weapons. the proper disposition therefore, of obstacles, is in advance of the ditch within short musket range.
Marshes, water courses, wet ditches, precipices, &c., may be regarded as obstacles, if they are sufficient in themselves to stop the enemy’s progress. But, however strong, they are not solely to be relied on, as the strongest natural position may be carried if not vigilantly guarded.
In placing the ground around a work in a defensive attitude, every means should be taken to reduce the smallest possible number of the points by which the enemy may approach; so that, by accumulating the troops on the weak points, a more vigorous defense may be made. In making this arrangement, equal care should be given to everything, affording a shelter to the enemy, would enable him to approach the work unexposed to its fires. To prevent this, all hollow roads, or dry ditches, which are not enfiladed by the principal works, should be filled up, or else be watched by a detachment, covered by an advanced work. All trees, underwood, hedges, enclosures, and houses, within cannon range, should be cut down and leveled, and no stumps be allowed higher than two feet. Trees beyond cannon range should not be felled; or, if felled, they should be burnt, to prevent the enemy’s movements being concealed.
The military “truism” here – an obstacle not under the watchful eyes of a defender is not an obstacle… and may even be an avenue.
The comment about trees beyond cannon range should be placed in some context, I think. Mahan’s notion of tree lines might be that of a wood-lot or such where the undergrowth was minimal (ah… the days of free ranging livestock). Regardless his advice was to reduce any cover given the enemy, even if that meant clearing a wide view-shed around the fortification. Common sense at play again. However, practical application of such translated to a lot of tree cutting… and in some cases displacing civilians and removing their dwellings.
But there are some things that might lay within viewing distance of the fort which should not be removed:
If there are approaches, such as permanent bridges, fords, and roads, which may be equally serviceable to the assailed and to the enemy, they should be guarded with peculiar care; and be exposed to the enfilading fire of a work especially erected for their defense.
We are thus back to “greatly cut up by the fire of the works.” And often we see in the plan for Civil War fortifications the provision of additional bastions or works just to provide fire on a crossing point. Redoubt Brannan, in the sprawling Fortress Rosecrans at Murfreesboro, served such a purpose. On the south side of the fort, Lunette Thomas covered a railroad and turnpike approach to the fortress. And circling back to the question of dwellings in view, many homes and buildings in front of the works in Murfreesboro proper were left in place. But Federal officers had orders to shell and burn the buildings should any Confederate provocation arise. None did, but it is interesting how sort of a “risk based decision” was made in that regard.
But let us direct this discussion of obstacles to something specific and detailed. There were types of “accessories” which the defender might select to construct as obstacles….
The principal artificial obstacles are trous-de-loup, or military pits; abattis; palisades; fraises; stockades; chevaux-de-frise; small pickets; entanglements; crows-feet; inundations; and mines.
So, next up…. Trous-de-loup and abattis!
(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 43-4.)