Fortification Friday: Mahan’s nine principles which regulate “intrenchments”

Last Friday we considered Mahan’s observation that the concepts for attack and defense, when laid out for consideration, are inherently connected – symbiotic in a manner of speaking.  This connection provides some criteria from which the quality of the defense may be evaluated.  Furthermore, from that relation Mahan was able to derive some principles “which regulate the plan and profile of intrenchments” – nine of them to be exact.  Some were technical specifications, others spoke more to the operational nature of the defense.  These were:

  1. A flanked disposition should be the basis of the plan of all intrenchments.
  2. Every angle of defense should be 90°.
  3. A line of defense should not exceed 160 yards.
  4. A salient angle should not be less than 60º.
  5. A strong profile is essential to a vigorous defense.
  6. The bayonet should be chiefly relied on to repel the enemy.
  7. Intrenchements should be arranged to facilitate sorties.
  8. Intrenchments should contain a reserve proportioned to their importance.
  9. Intrenchments should be defended to the last extremity.

Let’s consider these in detail.

As a refresher, recall the location of flanks on the fort’s trace:


Those flanks provided the “base” from which fires covered the ground in front of the curtain… and thus the fortification itself.  In Mahan’s words, “The flanks sweep with fire the ground in front of the faces; remove sectors without fire and dead angles; cross their fire in front of the salients; and take the enemy’s column in flank.”  Recall here this “sweep” of fire from the flanks is defined as the Angle of Defense:


To maximize this sweep, Mahan called for a right angle.  “An acute angle of defense exposes the faces to the fire of the flanks; an obtuse angle leaves a portion of the ground in front of the face undefended.”  With properly set angles of defense, the faces provide a cross fire upon the line of defense:


Mahan insisted this line not be set too far or too close from the defenses.  This was governed by the range of musketry at the time, pre-Civil War.  He wrote, “A close fire of musketry is more deadly than one of artillery; the musket will kill at distances between 250 and 300 yards, but its fire is not very certain beyond 160 yards….”  And he added that the attacker’s primary weapon would be similar muskets with similar effects on the defender.

The last technical specification referred to the salient angles:


We see the opposite of the salient angle is the sector without fire, which is also the attacker’s preferred axis of attack.  Mahan’s preference called for a 60º angle:

A salient angle less than 60º is too weak to withstand the effects of weather; the interior space which it encloses is too confined for the manœûvers of the troops; it forms a large sector without fire in front of it; the faces of acute salients are, from their position, more exposed to the enemy’s enfilading fire than when the angle is obtuse.

Very specific technical considerations.

Those established, Mahan turned to the operational aspects, calling for a vigorous defense.   He observed that losses incurred by the attacker, “unless detained by obstacles in front of the ditch“, are so small that the real defense of the work must be made between the crests of the counterscarp and the parapet. Crossing the ditch would fatigue the attacker.  And scaling the scarp and parapet more so.  That done, Mahan stressed, “… the enemy presents himself in a fatigued and exhausted state to the bayonets of the assailed, who have mounted on top of their parapet to meet and drive him back into the ditch.”

Yes, the bayonet!  “Unless the assailed are determined to meet the enemy at the point of the bayonet, they must evacuate their works so soon as he has entered the ditch….”  A grim statement, but a point of fact.  Mahan responded that the bayonet was “the surest method of repelling the enemy.”  The use of the bayonet when used by a rested, well positioned defender had a moral effect upon the fatigued attacker.

But confronting the attacker with the bayonet was just one facet of repulsing the attack.  The full response required attacks on the flanks at that point of attack.  “A sortie made on the flanks of the enemy, at the moment when his column is either checked by the fire of the works or is in a state of disorder when entering the ditch, will generally prove decisive in repelling his attack.”

However, Mahan pointed out, such sortie attacks necessitated a large body of defenders.  In addition to those designated for couter-attack sorties, the defender needed a reserve “… to charge the enemy in any critical moment of disorder, and to cover the retreat of the troops if driven from the parapet.”

But while the defender should be ready in case the parapet was lost, the defender had have the resolve to hold that parapet.

The chief object of intrenchments is to enable the assailed to meet the enemy with success… This object can only be attained by defending the works to the last extremity; and unless attained, intrenchments would serve little other purpose than to shelter the assauled from the enemy’s fire; for the damage received by the enemy from the fire of troops who see safety only in retreat, and not in a courageous effort to repel the assault, will be necessarily trifling.

Look beyond the “bravado” and elan message here. In practical terms what Mahan is advising is the use of works that, by nature of their layout or distance from the main line of defense, would not be defended.  It would present extra work for the defender, but more importantly would necessitate difficult operations in front of the enemy’s attack.

As to retreating from one line to another, one of two things must happen; either the first line must be evacuated, before the enemy enters the ditch, in order that the assailed may gain their second line in safety; in which case the first line will be of little service; or else, if the assailed wait until the assault is made on the parapet before retreating, the enemy will enter pell-mell with them into the second line, which will thus be of no service.

More to that point:

Troops in action cannot enter into the spirit of fortification on paper.  A retreat carries with it all the moral effects of a defeat; it inspires the assailant; renders the retreating corps timid; and impairs the confidence of the troops of the second line in the strength of their own position, when they see the first line carried with such ease.  Add to this, the confusion that must ensue among the best disciplined troops, under such circumstances, and the importance attached to the principle will be fully justified.

These nine principles are important for us, working as historians, when assessing military operations during the Civil War.  Mahan’s teachings were the references from which most officers (even those non-West Pointers) worked when planning a defense.  Any interpretation of how well or poorly a particular leader or unit defended a point should use these nine principles as a measure.

I would also add these principles have a similar, direct application when “walking the fields.”  Particular those first four principles help answer the question “how were these works used?” (And… not to hit this point softly, in some cases even “are these earthworks?”)

(Citations from Dennis Hart Mahan, A Treatise on Field Fortifications, New York: John Wiley, 1852, pages 6-10.)

Published by Craig Swain

"Historical marker hunter" and Civil War enthusiast.

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