Fortification Friday: Relief of Intrenchments… talking elevation, not reinforcements!

Back from the holidays and getting back to the Friday installments with titles prompting double-takes from those with dirty minds…..

For the first three chapters of the Treatise on Field Fortifications, Mahan’s focus was on the construction and arrangement of the works without any detailed discussion of the external factors governing the layout of the works.  Yes, as if the works were build in a perfectly flat table top in conditions one would never see in real life.  But Mahan was writing a instructional text, so abstracting out reality for a bit was necessary to get the basics across.

Chapter four brought us to some of that “reality,” with Mahan discussing the impact of relief on the design of the works …. and that is “relief” in the sense of terrain elevations.   Sort of a new dimension to consider, beyond just horizontal and vertical, profiles and traces:

When a work is placed on level ground, it usually receives a uniform relief; but when the site is irregular, or there are commanding eminences within cannon range, a uniformity of relief cannot be preserved, because it might expose the interior of the work to the enemy’s view, from the commanding points.

Let us be fair, practical, and realistic here.  Excepting perhaps some seacoast defense, where field fortifications are not apt to be used, anything worth protecting with earthworks is probably within cannon range of some eminence or is sited on irregular terrain.  I’ve often marveled at the notion of “regular” terrain… as most terrain one will encounter is broken and, well, irregular.  So why call it irregular?  But I digress. The bottom line is that where the terrain did not provide a perfect, flat playground for the engineer to design his works, the earthwork had to consider those irregularities.

Mahan continued with the inputs needed to the fortification plan:

The plan will also be modified by the same causes. The principal faces should be placed as not only to guard all the points where an enemy might approach, but the enemy should not be able to take up their prolongations to obtain an enfilading, or a reverse fire upon them.  The position of the points to be guarded, and that of the commanding eminences, require to be carefully studied before adopting any definitive plans.

So… one should take into account these irregularities of terrain, in particular any place the enemy might gain an elevation advantage, before reaching a final plan for the earthworks.  Sounds simple.  But again we are at one of those points with military science where common sense sounds so simple but is darn difficult to apply.   Put yourself at, say, Harper’s Ferry with an adversary holding Maryland and Loudoun Heights.  Now try to apply the common sense espoused in the citation above.  Not like one can wave a hand and fix those problems.

How to remedy such sticky problems?  Some general guidance and “rules of thumb” to follow:

The only general rules that can be laid down, are to lay out the principal lines so as to obtain a direct and cross fire on the approaches of the enemy, and placing them, at the same time, as nearly parallel as practical, to the general direction of the crests of the commanding heights, in order that the enemy occupying the crest may have a direct fire along on these parts.

So this is some relief (the refreshing kind of relief, that is) to the poor engineer sitting in Harper’s Ferry.   But just a little.  By negating any advantage of flanking, enfilading, or reverse fires, the firepower arithmetic is simpler.  Though that still does not turn around the advantage of commanding heights, as Mahan spoke of next:

When the enemy occupies a position more elevated than the work, he is said to have a plunging fire on it; and when the relief of the work is so regulated as to intercept this fire, the work is said to be defiled.

I like this passage.  We see the problem defined and labeled as plunging fire.  Then we see the textbook remediation that can solve the problem… pile dirt higher!

Well not just pile dirt higher.  Rather think of ways to put dirt, rock, or other obstructions between the defender and any enemy on those elevations.  However, Mahan continued with a cautionary note that plans should not get carried away in this regard:

The defilement of field works is not indispensable to a good defense; nor is it generally practicable. It is, however, not only a conservative means, but it also inspires the assailed with confidence; for the soldier regards with distrust the strength of his position, when he finds himself exposed to the view of the enemy from an elevated point.

So call it an “optional” facet to the planning, but one that one should strongly consider picking up.

Mahan followed this up with a “practical example” of how to plan a defilement.  We’ll look at that in the next installment.  For now, consider again the play here of something sensible within military science.  It is very easy to stand at some spot and say “this is a bad spot as it is dominated by the high ground.”  But that assessment must consider that the defender of that “bad spot” was probably tied to that location by situational necessity.  It was, perhaps, a point of such value that the defense had to be made.  With that in mind, we really must be considering how vigorously and rigorously the defender worked to turn a “bad spot” into something at least a little less bad.

(Citation from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, page 24-5.)


3 thoughts on “Fortification Friday: Relief of Intrenchments… talking elevation, not reinforcements!

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