Investment is the military tactic of surrounding an enemy fort (or town) with armed forces to prevent entry or escape.

A circumvallation is a line of fortifications, built by the attackers around the besieged

Siege

A siege is a military blockade of a city or fortress with the intent of conquering by attrition or assault. The term derives from sedere, Latin for "to sit". Generally speaking, siege warfare is a form of constant, low intensity conflict characterized by one party holding a strong, static...

 fortification facing towards the enemy fort (to protect the beseigers from sortie

Sortie

Sortie is a term for deployment or dispatch of one military unit, be it an aircraft, ship, or troops from a strongpoint. The sortie, whether by one or more aircraft or vessels, usually has a specific mission....

s by its defenders and to enhance the blockade

Blockade

A blockade is an effort to cut off food, supplies, war material or communications from a particular area by force, either in part or totally. A blockade should not be confused with an embargo or sanctions, which are legal barriers to trade, and is distinct from a siege in that a blockade is usually...

). The resulting fortifications are known as 'lines of circumvallation'. Lines of circumvallation generally consist of earthen rampart

Defensive wall

A defensive wall is a fortification used to protect a city or settlement from potential aggressors. In ancient to modern times, they were used to enclose settlements...

s and entrenchment

Entrenchment

Entrenchment or Entrenched may refer to:* Military trenches with relation to Trench warfare, especially that of World War I *Entrenchment as a feature of a Constitution, making it more difficult to amend* Entrenched clause within a constitution...

s 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 the enemy fort. This is a second line of fortifications outside the circumvallation, facing away from the enemy fort. The contravallation protects the besiegers from attacks by allies of the city's defenders and enhances the blockade

Blockade

A blockade is an effort to cut off food, supplies, war material or communications from a particular area by force, either in part or totally. A blockade should not be confused with an embargo or sanctions, which are legal barriers to trade, and is distinct from a siege in that a blockade is usually...

 of the enemy fort by making it more difficult to smuggle in supplies.
The Siege of Alesia which took place in September 52 BC

52 BC

Year 52 BC was a year of the pre-Julian Roman calendar. At the time, it was known as the Year of the Consulship of Pompeius and Scipio...

 is one of the most famous investments in history. Julius Caesar

Julius Caesar

Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 in his Commentaries on the Gallic War describes his textbook use of the circumvallation and contravallation to defeat the Gaul

Gaul

Gaul was a region of Western Europe during the Iron Age and Roman era, encompassing present day France, Luxembourg and Belgium, most of Switzerland, the western part of Northern Italy, as well as the parts of the Netherlands and Germany on the left bank of the Rhine. The Gauls were the speakers of...

s under their chieftain Vercingetorix

Vercingetorix

Vercingetorix was the chieftain of the Arverni tribe, who united the Gauls in an ultimately unsuccessful revolt against Roman forces during the last phase of Julius Caesar's Gallic Wars....

.

The basic objectives and tactics of a military investment have remained the same down to today. During the Second World War

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 there were many sieges and many investments. One of the most famous sieges of the Second World War which demonstrated the tactical use of investment was the siege of Stalingrad

Battle of Stalingrad

The Battle of Stalingrad was a major battle of World War II in which Nazi Germany and its allies fought the Soviet Union for control of the city of Stalingrad in southwestern Russia. The battle took place between 23 August 1942 and 2 February 1943...

. 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 into the city 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

Airbridge (logistics)

An airbridge is the route and means of delivering material from one place to another by an airlift.An airbridge is the means by which an airhead is kept supplied by overflying enemy held territory...

) eventually forced the starving Germans inside the city to surrender.