Siege Works: Lines of Circumvallation and Countervallation

Civil War Field Fortifications

Lines of Circumvallation and Countervallation

A line of circumvallation was originally understood as a continuous line of field fortifications that protected a besieging army's camps and artillery parks from outside enemy forces attempting to assist the besieged garrison. Until the late Eighteenth century some important European fortress cities were still small enough to make the investment of time, labor, and material worthwhile for a besieging army to enclose its rear with lines of circumvallation. Expansion of the physical area occupied by cities and the decline of stone and masonry fortifications combined with the increase in numbers of troops composing field armies in the early Nineteenth century to make continuous lines of circumvallation a rather too expensive luxury that most armies could not afford. These static lines of works were replaced with a more mobile expedient: armies of observation. An army of observation served much the same purpose as a line of circumvallation: its objective was to watch over and protect the rear of the besieging army from enemy attempts to run supplies through to the besieged garrison or to relieve the garrison by driving the besieging army away from the fortified place. An army of observation had the advantage of being able to intercept an enemy army of relief at greater distances from the fortified place, thus making it much more difficult for a besieged garrison to coordinate its activities, particularly sorties to distract the besieging army as the relieving army approached, with those of the relieving army.

Even so, lines of circumvallation actually did make an occasional appearance in the American Civil War, though they were just a shadow of the original idea of all encompassing lines. At the Siege of Petersburg, Virginia the Federal army found it both necessary and desirable to protect its left flank and rear with a solid and well built line of works that stretched from Blackwater Swamp to Fort Dushane on the Weldon Railroad and on to Fort Cummings on the Squirrel Level Road. This line protected the rear of the Federal lines facing the Confederate works around Petersburg and formed a definite, if incomplete, line of circumvallation that opposed Confederate mobility with strong and expensive earthworks. The Federal army besieging Vicksburg found it necessary to protect its rear by detaching part of its force as an army of observation to interfere with the Confederate forces based at Jackson, Mississippi efforts to relieve the besieged garrison. This army of observation covered the direct route from Jackson to Vicksburg at the Big Black River with a line of light field works that the Confederates would have found difficult to break. While it seems doubtful whether this sort of line can be called a line of circumvallation, it served the same purpose and protected besieging army's rear from outside enemy intervention.

Lines of countervallation continued to play a useful and natural part in sieges during the Civil War. Where ever an army protected itself and a strategic point with a line of earthworks, the attacking army answered the challenge by digging its own line of works. Lines of countervallation were composed of defensive field fortifications that protected the besieging army's position from the besieged garrison's attempts to impeded the progress of the siege by launching sorties against important points on the flanks of the besieging army's approaches or against his camps and depots. The Federal line at Petersburg that stretched from the Appomattox River and Fort McGilvery on the right to Fort Gregg on the left may be considered a line of countervallation that engaged the main Confederate line around Petersburg. This line provided a solid base that allowed the Federal army to advance its offensive siege works, such as Burnside's infamous mine, against the Confederate line. At Vicksburg the Federal army entrenched a line along a series of ridges opposite the Confederate line and created a defensive position that was every bit as unassailable as the Confederate's own line before they pushed their approaches forward from that line.

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