Area denial

An area denial weapon is a device used to prevent an adversary from occupying or traversing an area of land, sea or air (Anti Access/Area Denial or A2/AD). The specific method used does not have to be totally effective in preventing passage (and sometimes is not) as long as it is sufficient to severely restrict, slow down, or endanger the opponent. Some area denial weapons pose long-lasting risks to anyone entering the area, specifically to civilians, and thus are often controversial.

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 advantage 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.

A more modern version, allowing quicker dispersal and providing the advantage of being hidden more easily, are caltrops, though items bearing close similarity (small balls with spikes) had been in use for most of antiquity. Many variants were also used, such as boards with metal hooks, as described during battles of Julius Caesar.

Passive fortification—ditches and obstacles such as dragon's teeth and Czech hedgehogs—were used as anti-tank (the modern "cavalry") measures during World War II.

Simple rows or clusters of sharpened sticks (nowadays also known as punji sticks), and the use of small caltrops have been a feature of anti-infantry 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. In modern times, special caltrops are also sometimes used against wheeled vehicles with pneumatic tires. Some South American urban guerrillas as the Tupamaros and Montoneros called them "miguelitos" and used these as a tactic to avoid pursuit after ambushes.

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