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Investment is the military process of surrounding an enemyfort (ortown) witharmed forces to prevent entry or escape.[1][2] It serves both to cut communications with the outside world and to prevent supplies and reinforcements from being introduced.
Acontravallation is a line of fortifications built by the attackers around thebesieged fortification facing towards an enemy fort to protect the besiegers fromsorties by its defenders and to enhance theblockade.[3][verification needed][4] The contravallation can be used as a base to launch assaults against the besieged city or to construct further earthworks nearer to the city.
Acircumvallation may be constructed if the besieging army is threatened by a field army allied to an enemy fort. It is a second line of fortifications outside the contravallation that faces away from an enemy fort. The circumvallation protects the besiegers from attacks by allies of the city's defenders and enhances theblockade of an enemy fort by making it more difficult to smuggle in supplies.[5]
Lines of contravallation and circumvallation generally consist of earthenramparts andentrenchments that encircle the besieged city.
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Thucydides notes the role circumvallation played in theSicilian Expedition and in the Spartansiege ofPlataea during the initial stages of thePeloponnesian War in 429 BC.
Julius Caesar in hisCommentaries on the Gallic War describes his textbook use of the circumvallation[6] to defeat theGauls under their chieftain,Vercingetorix, at theSiege of Alesia in September 52 BC.
During theSiege of Jerusalem, Titus and his Roman legions built a circumvallation, cutting down all trees within fifteen kilometres (9 miles).
Another example from the pre-modern period is theSiege of Constantinople (717–718).
Thecaliph of theUmayyad Empire took advantage of the violent anarchy in theByzantine Empire to prepare a huge host, comprising more than 100,000 troops and 1,800 ships, to take them to the Byzantine capital,Constantinople. Upon arriving outside the city'sTheodosian walls, the Arab army had some knowledge that EmperorLeo III the Isaurian had allied withBulgaria underKhanTervel, and so, in preparation for theBulgarian army, built a set of stone walls against the city and against the countryside, with the Arab camp in between.[7]
KingPepin the Short ofFrancia built a number of fortified camps during hisSiege of Bourbon (761) to surround the town completely.[8] He built a complete set of lines of circumvallation and contravallation during theSiege of Bourges (762).[9]
The basic objectives and tactics of a military investment have remained the same in the modern era. During theSecond World War, there were many sieges and many investments. One of the best-known sieges of the war, which demonstrated the tactical use of investment, was theSiege of Stalingrad. During the first half of the siege, the Germans were unable to fully encircle the city and so the Soviets got men and supplies in across theVolga River. During the second half of the battle, the complete investment of Stalingrad by the Soviets, including airspace, which prevented the construction by the Germans of an adequately largeairbridge, eventually forced the starving Germans in the city to surrender.
In modern times, investments andsieges of cities are often combined with intensiveshelling,air strikes and extensive use ofland and/orsea mines.
If an attempt at relief from without was to be feared, another line of works must be created, outside the first, and facing outwards. In modern warfare this latter line is called thecircumvallation, and the inner one thecontravallation.
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