Redoubt

Also Redout, Redoute (french). In the context of nineteenth century military engineering the term redoubt referred to very different types of fortifications that served very different functions depending on its frame of reference. Taken in the context of permanent fortification a redoubt was a reduit within a larger outwork; in the context of field fortification a redoubt referred to a particular type of field work defined by its outline or trace. In common usage the term has often been used to denote a field work of unusual strength.

1. Permanent Fortifications

A redoubt in permanent fortifications was a reduit within an outwork designed to prolong the defense of the work after its scarp had been breached or to provide an interior shelter for the collection of troops and materials necessary for the defense of the outwork. In most cases redoubts were profiled to give the crest of the interior slope of their parapet a command of fire over the parapet of the work itself and the ground in front of it to allow the redoubt to contribute its fire in opposition to the advance of an attacker's siege works. In Cormontaingne's method of tracing a bastion front of fortification (shown in the illustration) the demi-lune redoubt (A) controls the interior of the demi-lune, adds its fire to that of the demi-lune faces and secures communication between the demi-lune and enceinte. The redoubts of the re-entering places of arms (B) provided a secure area where troops could be formed for the defense of the covered way or for sorties; its fire could also support sorties. It enfiladed the covered way and could add its fire to impede the crowning of the covered way. Since the redoubt also had a slightly oblique reverse view of the face of the demi-lune its fire could oppose both an attacker's advance across the ditch of the demi-lune and his attempt to form a lodgment in a breach in the demi-lune scarp.

2. Field Fortifications

In field fortifications a redoubt was an enclosed detached field work without re-entering angles. Redoubts could be laid out as any regular or irregular convex polygon as necessary to adequately fortify a particular site; the most common being four and five sided figures. This type of field work could be adapted to fortify almost any position, but were generally applied to situations in which a garrison might be compelled to defend itself against attacks from any direction. This included isolated garrison posts along lines of communication and continuous lines of works where it might be necessary for a work's garrison to maintain its position after another section of the line had been breached. Several redoubts could be placed in defensive relation to each other to form lines with intervals in which fire from the faces of the redoubts crossed to defend open intervals between the works. In some systems of attack by regular approaches redoubts were prescribed at the extremities of the various parallels to protect advancing siege works from sorties.

Although redoubts had the great advantage of being easily adapted to all sorts of terrain, they also enjoyed a number of more or less serious defects. Most importantly, the ditch could not be defended by flanking fire delivered from the parapet of the work. Without accessory means of defense within the ditch itself, an attacking body of troops in the ditch was safe from the defenders' fire and could re-organize to scale the scarp and mount the parapet. Each salient angle produced a wide sector without fire on either side of the prolongation of its capital which could not be defended by direct fire from the parapet. Unless accessory means of defense were provided by the interior arrangements of a redoubt an attacking body of troops could advance along the capitals of the salients without being subjected to a close and destructive fire from the work's parapet. Since all angles were salient and pointed toward the exterior side of the outline, the faces could be subject to enfilade fire delivered from batteries or groups of riflemen positioned at a distance from the work.

All of these defects could be partially mitigated by inclusion of accessory means of defense and interior arrangements. If a redoubt defended a particularly important position its ditch could be defended by caponnieres or counterscarp galleries that allowed the defenders to bring a close flanking fire along the full length of the bottom of the ditch. Either of these structures in the ditch itself could prevent an attacking body of troops from resting and re-organizing within the ditch and would most certainly compel attacking troops to assault the ditch defenses prior to making an attempt to scale the scarp. A garrison provided with hand grenades or specially fused artillery shells that could be rolled or thrown over the parapet could defend the ditch and harass attacking troops without necessity for the defenders to actually be in the ditch itself.

Sectors without fire in front of the salients could be eliminated by forming pan coupés along the interior slopes of the parapet within the salients. This would allow the defenders to bring a direct fire along the capitals of the salients. It was common to position artillery at the pan coupes of the salients mounted either en barbette to give a wide field of fire or en embrasure to fire directly along the capital. Some engineering manuals suggested cutting the interior slope of the parapet en crémaillère on either side of the salient to allow defenders to deliver direct fire parallel to the capital of the salients, but this rarely done since it increased the time, labor, and difficulty of completing the construction a redoubt and was not considered to be particularly effective since it reduced the volume of direct fire that could be projected from a redoubt's faces on either side of the salient.

The threat of enfilade fire along the interior side of a redoubt's face could be mitigated by adequate defilading when the work was laid out on the ground and constructed. Prolongations of the lines of a redoubts interior crest had to be directed toward ground that an attacking force could not easily occupy or readily establish batteries capable of firing at elevations necessary for ricocheting a redoubt's banquettes. If this was not possible, and it often was not, the interior side of the parapet could be broken with bomb or splinter proof traverses designed to intercept enfilade fire. Traverses tended to reduce the number of troops who could arm a parapet and therefore interfered with the amount of fire that could be delivered in defense of a redoubt, but a reduction in fire was better than allowing troops to suffer a loss of confidence in the protective function of their fortification.

As with all detached enclosed field works the length of a redoubt's parapet and area it enclosed were governed by the number of troops and pieces of artillery composing its garrison and type of interior structures necessary for defense of the work. Total length of parapet required for a particular garrison could be roughly calculated under the assumption that each file (one or two men) of troops arming a parapet occupied one linear yard along the line of the interior crest and that each gun occupied five or six linear yards. Taking the total number of troops (reduced by 1/3 to 1/4 to allow for a reserve that would not initially occupy positions on the parapet) and dividing it by the number of files gave the length of parapet required for the small arms armament of the redoubt. The total number of guns to be mounted in the work was multiplied by the linear yards required for each gun. When added together these two numbers represented the length, in linear yards, required to give the work's garrison adequate space along the interior crest for its defense. Structures such as barbette platforms and traverses that connected to the interior side of the parapet would also have to be added in to determine the total linear yards of interior crest necessary for the work's intended garrison.

It was also assumed that the garrison of an enclosed field work might be compelled to remain within the work for several days at a stretch when under attack by a determined enemy. Consequently enclosed works had to have sufficient interior space on the terre-parade to house their garrisons. The area required by a garrison was determined by allowing 1.5 square yards for each man and about 60 square yards for each gun (including its caisson and limber). A garrison consisting of 200 men and 4 guns would require at least 540 square yards ((1.5 x 200)+ (4 x 60)) of unencumbered space within the work. Interior space rarely presented a serious design problem unless a redoubt was extremely small (with faces less than 20 yards long) or the interior of the work was cluttered with structures that could not be used to shelter the garrison.

3. Common Usage

The term redoubt was also often used in a very non-technical, but very traditional, sense to denote field works of great strength, regardless of their actual outlines. The Great Redoubt, constructed by the Russians to strengthen their position prior to the Battle of Borodino in 1812, had the outline of a common lunette with auxiliary flanks. Although its name may suggest otherwise, the Great Redoubt was a well sited open detached field work that withstood numerous assaults before French cavalry entered and captured the work through its unprotected gorge. Many works constructed during the American Civil War were also named to imply strength rather than as a means of exact identification of a particular type of field work. Neither the Railroad Redoubt nor the Great Redoubt at Vicksburg, for example, were enclosed field works without re-entering angles. The former did have the advantage of being an irregular work enclosed on three sides, but the latter was little more than a rather indefinite section of rifle trenches and batteries traced on an irregular line along a commanding ridge. It is important to note that this usage tends to take hold after an event has given a strong field work some historical interest that suggests that the work requires a name to match its effectiveness. These post-event field work names do not always correspond with the names that participants in an event actually used to identify a work.

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October, 2003

September, 2005