Investment is the military process of surrounding an enemy fort (or town) with armed forces to prevent entry or escape. It serves both to cut communications with the outside world, and to prevent supplies and reinforcements from being introduced.
A circumvallation is a line of fortifications, built by the attackers around the besieged fortification facing towards an enemy fort (to protect the besiegers from sorties by its defenders and to enhance the blockade). The resulting fortifications are known as 'lines of circumvallation'. Lines of circumvallation generally consist of earthen ramparts and entrenchments that encircle the besieged city. The line of circumvallation can be used as a base for launching assaults against the besieged city or for constructing further earthworks nearer to the city.
A contravallation may be constructed in cases where the besieging army is threatened by a field army allied to an enemy fort. This is a second line of fortifications outside the circumvallation, facing away from an enemy fort. The contravallation protects the besiegers from attacks by allies of the city's defenders and enhances the blockade of an enemy fort by making it more difficult to smuggle in supplies.
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Julius Caesar in his Commentaries on the Gallic War describes his textbook use of the circumvallation and contravallation to defeat the Gauls under their chieftain Vercingetorix at the Siege of Alesia in September 52 BC.
Another example from the pre-modern period is the siege of Constantinople in AD 717–718. The leaders of the Islamic Empire took advantage of the violent anarchy within the Byzantine state to prepare a huge host, comprising more than 100,000 troops and 1,800 ships, to take them to the capital Constantinople. Upon arriving outside the Theodosian walls, the Arab host had some knowledge that the Emperor Leo III the Isaurian had allied with Bulgaria under their khan Tervel, and so in preparation for the Bulgarian army, built a set of stone walls against the city and against the countryside, with the Arab camp in between.
King Pepin the Short of Francia built a number of fortified camps during his Siege of Bourbon (761) to completely surround the town. He built a complete set of lines of circumvallation and contravallation during the Siege of Bourges (762).
The basic objectives and tactics of a military investment have remained the same in the modern era. During the Second World War there were many sieges and many investments. One of the best known sieges of World War II, which demonstrated the tactical use of investment, was the siege of Stalingrad. During the first half of the siege the Germans were unable to fully encircle the city, so the Soviets were able to get men and supplies in across the Volga River. In the second half of the battle, the complete investment of Stalingrad by the Soviets (including air space which prevented the construction by the Germans of an adequately large airbridge) eventually forced the starving Germans inside the city to surrender.