Area denial weapons

Area denial weapons are used to prevent an adversary from occupying or traversing an area of land. The method does not have to be totally effective (and usually isn't) as long as it is sufficient to severely restrict, slow down, or endanger the opponent. Most area denial weapons pose long-lasting risks to anyone entering the area, specifically to civilians, and thus are often controversial.

Anti-cavalry
In medieval warfare, sharp and sturdy stakes were buried at the bottom of long lines of ditches, pointed end up diagonally, in order to prevent cavalry charges in a given area. Even if the stakes were spotted, soldiers would be forced to dismount and effectively give up their purpose as cavalry as well as becoming easier targets. The correct layout of these extensive lines of ditches and the quality control of stake size, form and placement was part of the craft of war. Many simpler variants were also used, such as caltrops and boards with metal hooks, as described during battles of Julius Caesar.

Similar passive fortifications—ditches and obstacles such as dragon's teeth and Czech hedgehogs—were used as anti-tank defences during World War II.

Anti-infantry
Simple rows or clusters of sharpened sticks - the ancient equivalent of modern-day punji sticks - and the use of small caltrops against infantry, have been a feature of warfare for a long time. However, due to the difficulty of mass-producing them in the pre-modern age, they were rarely used except in the defense of limited areas or chokepoints, especially during sieges, where they were used to help seal breaches. Increasing ease of production still did not prevent these methods from slowly falling out of favor from the late Middle Ages onward.

Caltrops are still sometimes used in modern conflicts, such as during the Korean War, where Chinese troops, often wearing only light shoes, were particularly vulnerable.

Explosives
The most common are land mines of various types, planted by hand or dispersed by artillery. Some modern prototypes experiment with automatic guns or artillery-delivered ammunitions that are fired only after remote sensing detects enemies.

Booby traps or improvised explosive devices in sufficient concentration also qualify as area denial weapons, though they are much easier to clear and usually pose less long-term danger.

NBC agents
Various NBC (nuclear, biological, chemical) weapons can be used for area denial, as long as the agent is long-lasting. Fallout from nuclear weapons might be used in such a role. While never actually employed in this form, its use had been suggested by Douglas MacArthur during the Korean War.

Anthrax spores can contaminate ground for long times, thus providing a form of area denial. However, the short-term (tactical) effects are likely to be low - the psychological effects on an opponent would likely be more significant.

The massive use of defoliants such as Agent Orange can be used as an interdiction measure because they leave areas empty of any form of vegetation cover. In the desert-like terrain that ensues, it is impossible for an adversary to travel without being seen, and there is little cover in case of an attack, especially from the air.

Many chemical weapons also produce toxic effects on any personnel in an affected area. However, this usually has no tactical value, as the effects of indirect exposure do not develop fast or substantially enough - though again, the psychological effect upon an enemy aware of the chemical usage may be considerable. There are however some chemical agents that are by design non-degrading, such as the nerve agent VX.

Targeted
To address some of the problems with land-mines (see 'Drawbacks'), weapons manufacturers are now experimenting with area-denial weapons which need human command to operate. Such systems are usually envisioned as a combination of either explosives, pre-targeted artillery shelling or smartguns with remote sensing equipment (sound, vibration, sight/thermal). By not posing a long-term risk, and by having some level of IFF capability (automatic or human-decision-based), these systems aim to achieve compliance with the Ottawa Treaty, as for example the Metal Storm ADWS (Area Denial Weapons System).

Drawbacks
As area denial weapons do not discriminate between friend and foe (or civilians) they make the affected zone hazardous for all trying to enter. Concepts for area denial weapons which do discriminate (by active sensing) have often been proposed, but have not yet reached a stage of general usefulness, due to their high complexity (and cost) and the risk of misidentification.

Explosive-based area-denial weapons (mines) may be intentionally equipped with detonators which degrade over time, either exploding them or rendering them (relatively) harmless. Even in these cases, unexploded munitions often pose significant risk. Even though the percentage of permanently unsafe munitions may be in the low single-digit percentages, the sheer number of munitions ('bomblets') dispersed by weapons like cluster bombs still pose substantial risks.