Civil War Field Fortifications
Mahan's Principles for the Design and Defense of Field Fortifications
The following nine principles were presented by D. H. Mahan in his A Complete Treatise on Field Fortifications published in 1836. The first four deal with elements that were necessary for the design of a defensible field work, the last five concern the manner in which field works were to be defended. Taken as a whole Mahan's principles provide a succinct view of the defensive purpose of the forms given field fortifications and the methods necessary for a successful defense using those forms.
1. "A flanked disposition should be the basis of the plan of all intrenchments."
The outlines of all field works were best arranged when flanking faces were included that covered the ground in front of the faces with their columns of fire. Flanks allowed the defenders to catch the enemy in cross fires as he advanced on the work and helped protect the sectors without fire in front of salient angles.
2. "Every angle of defense should be 90 degrees."
Angles of defense that were greater than 90 degrees placed the faces within the columns of fire of the flanks, those less than 90 degrees left ground in front of the work undefended by fire. 90 degree angles maximized the destructive effects of the defenders fire and allowed the flanks to adequately support the faces of a field work.
3. "A line of defense should not exceed 160 yards."
A good defense relied on crossed musket fire to repel an enemy attack before it reached the ditch of a field work. At the time Mahan wrote most infantry were armed with smoothbore muskets which were quite destructive at ranges below 160 yards. Mahan's idea was to make sure that the flanks of a field work could support the faces with close range musket fire for their entire length, to do this the faces had to be no more than 160 yards long.
4. "A salient angle should not be less than 60 degrees."
Angles less than 60 degrees were subject to rapid erosion, produced an interior space that was too small for troops to move freely, created large sectors without fire, and exposed the faces forming the salient to enfilade fire from within the sector without fire. In short, smaller angles produced weak points in a field work that were both difficult to maintain and negated the advantages of fighting from behind a strong parapet.
5. "A strong profile is essential to a vigorous defense."
Major field works were designed to both protect the garrison from enemy fire and to present a significant obstacle to an assault. After an attacking body of troops had crossed the obstacles in front of a work while under fire they encountered a deep ditch that might contain yet another obstacle and finally a high parapet that had to be climbed to reach a position to actually enter the work. By the time they reached the parapet their excitement for the attack was supposed to be cooled by fatigue and a general state of confusion that prohibited the attacking troops from acting in unison. In mounting the superior slope the exhausted attackers were vulnerable to a strong counter attack by the defending troops. The harder it was for the enemy to actually reach the superior slope, the easier they were to defeat once they made the final attempt to force their way into the work.
6. "The bayonet should be chiefly relied on to repel the enemy."
The decisive moment in an attack came when the enemy reached the superior slope and attempted to drive the defending troops back from the banquette. Supported by difficult obstacles and a strong profile, the defenders' best chance to repel the enemy was to immediately counter attack with fixed bayonets to chase the enemy off the parapet and away from the work. Troops who were unable or incapable of standing firm and countering attacking at the decisive moment stood little chance of being able to defend themselves once the enemy mounted the superior slope. Breaking to the rear at that moment would lead to their complete destruction, therefore weak troops had to be withdrawn from a field work as soon as the enemy entered the ditch, well before the decisive moment was attained.
7. "Intrenchments should be arranged to facilitate sorties."
When large infantry forces were involved in the defense of a field work or line of works intervals had to be left open between the works to allow reserves to advance against the enemy once he had become disordered and fatigued crossing the obstacles in front of the works. A strong counter attack was considered the best means for defeating the enemy and driving him away from the line of works. Strong counter attacks could not be launched over the same obstacles that were intended to disrupt and wear out an attacking body of troops without causing the same disruption and fatigue to the countering attacking troops. Small garrisons, on the other hand, were to be content with simply repelling the enemy from their fortification and were not advised to give up the protection of their work in the face of a larger enemy force.
8. "Intrenchments should contain a reserve proportioned to their importance."
A reserve was essential for the defense of a field work. Sheltered from enemy fire within a work, the reserve was fresh and ready to counter attack the enemy as soon as he mounted the superior slope. When troops lining the parapet were driven from the banquette the reserve could counter charge to drive the enemy back over the parapet and into the ditch.
9. "Intrenchments should be defended to the last extremity."
The whole purpose of a field fortification was to allow a defending force to meet the enemy under conditions that would be bound to promote a successful defense of a position. Any works that were designed to be given up at the enemy's approach, without a vigorous defense, could not contribute to the repulse of the enemy's attack. Once the troops temporarily holding such a work started to retire they gave up the moral advantage offered by the protection of the fortification. Once a retreat started and the moral ascendancy had been turned over to the enemy without a fight, stopping troops who had their backs toward the enemy it at a second line of works was, at best, a problematical question.
Transcriptions on the attack and defense of field fortifications
DuFour Duparcq Halleck Regulations Jebb Barnard Mahan-2
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