A caltrop (also known as caltrap, galtrop, cheval trap, galthrap, galtrap, calthrop, crow's foot) is an antipersonnel weapon made up of two or more sharp nails or spines arranged in such a manner that one of them always points upward from a stable base (for example, a tetrahedron). Caltrops were part of defenses that served to slow the advance of horses, war elephants, and human troops. They were said to be particularly effective against the soft feet of camels. In more modern times, caltrops are used against wheeled vehicles with pneumatic tires.
The modern name "caltrop" is derived from the Latin calcitrapa (foot-trap), like in the French chausse-trape. The synonymous Latin word tribulus gave rise to the modern Latin name of a plant offering similar hazards to sandaled or bare feet, Tribulus terrestris (Zygophyllaceae), whose spiked seed case can injure feet and puncture tires. This plant can also be compared to the starthistle, Centaurea calcitrapa, which is sometimes called the "caltrop". A water plant with similarly-shaped spiked seeds is called the "water caltrop", Trapa natans.
According to Quintus Curtius (IV.13.36), iron caltrops were used as early as 331 BC at Gaugamela. They were known to the Romans as tribulus or sometimes as murex ferreus, the latter meaning 'jagged iron'.
The armed chariots used in war by Antiochus and Mithridates at first terrified the Romans, but they afterwards made a jest of them. As a chariot of this sort does not always meet with plain and level ground, the least obstruction stops it. And if one of the horses be either killed or wounded, it falls into the enemy's hands. The Roman soldiers rendered them useless chiefly by the following contrivance: at the instant the engagement began, they strewed the field of battle with caltrops, and the horses that drew the chariots, running full speed on them, were infallibly destroyed. A caltrop is a device composed of four spikes or points arranged so that in whatever manner it is thrown on the ground, it rests on three and presents the fourth upright.
Contrary to popular belief, there is no evidence that the caltrop was used in either the first Wars of Scottish Independence (1296–1328), or second wars (1332–1357). A single example was found in Jamestown, Virginia, in the United States.
Undoubtedly the most unusual weapon or military device surviving from seventeenth-century Virginia is known as a caltrop, a single example of which has been found at Jamestown. It amounts to a widely spread iron tripod about three inches long with another leg sticking vertically upward, so that however you throw it down, one spike always sticks up. ... There is no doubt that the most inscrutable Indian treading on a caltrop would be shocked into noisy comment. ... The fact that only one has been found would seem to suggest that they were used little, if at all. As with all military equipment designed for European wars, the caltrop’s presence in Virginia must be considered in the light of possible attacks by the Spaniards as well as assaults from the Indians.
The Japanese version of the caltrop is called makibishi. Makibishi were sharp spiked objects that were used in feudal Japan to slow pursuers and also were used in the defence of samurai fortifications. Iron makibishi were called tetsubishi, while the makibishi made from the dried seed pod of the water chestnut, formed a natural type of makibashi called tennenbishi. Both types of makibishi could penetrate the thin soles of shoes, such as the waraji sandals, that were commonly worn in feudal Japan.
Caltrops were used extensively and to very good effect during World War II. The modifications and variants produced by the Special Operations Executive (SOE) and the Office of Strategic Services (OSS) of the United States  are still in use today within Special Forces and Law Enforcement bodies[who?].
Punji sticks perform a similar role to caltrops. These are sharpened sticks placed vertically in the ground. Their use in modern times targets the body and limbs of a falling victim by means of a pit or tripwire.
During the Second World War, large caltrop-shaped objects made from reinforced concrete were used as anti-tank devices, although it seems that these were rare. Much more common were concrete devices called dragon's teeth, which were designed to wedge into tank treads. Large ones weighing over 1 tonne (1.1 tons) are still used defensively by the army to deny access to wheeled vehicles, especially in camp areas. As dragon's teeth are immobile, the analogy with the caltrop is inexact. Another caltrop-like defence during World War II was the massive steel, freestanding Czech hedgehog; the works were designed as anti-tank obstacles, and could also damage landing craft and warships that came too close to shore. These were used by the Germans to defend beaches at Normandy and other coastal areas.
Tetrapods are concrete blocks shaped like caltrops, which interlock when piled up. They are used in the construction of breakwaters and other sea defences, as they have been found to let the water pass through them and interrupt natural processes less than some other defenses.
Tire deflation device
Australian Light Horse troops
During service in World War I, Australian Light Horse troops collected caltrops as keepsakes. These caltrops were either made by welding two pieces of wire together to form a four-pointed star or pouring molten steel into a mould to form a solid, seven-pointed star. The purpose of these devices was to disable horses. They were exchanged with French troops for bullets. The Australian Light Horse troops referred to them as "Horse Chestnuts".
Caltrops have been used at times during labor strikes and other disputes. Such devices were used by some to destroy the tires of management and replacement workers. In this context, caltrops are usually referred to as "jack rocks." They have become icons at pro-union rallies, often depicted on t-shirts, hats, or worn as jewelry in some cases.
A caltrop has had a variety of symbolic uses and is commonly found as a charge in heraldry. For instance, the Finnish noble family Fotangel (Swedish for 'caltrop') had arms Gules, three caltrops Argent.
It has been adopted by military units: the caltrop is the symbol of the US Army's III Corps, which is based at Fort Hood, Texas. III Corps traces its lineage to the days of horse cavalry, which used the caltrop as a defensive area denial weapon.
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- Czech hedgehog
- Spike strip
- Knucklebones, a game, with similar hazards
- Water caltrop, a plant
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