A caltrop (also known as caltrap, galtrop, cheval trap, galthrap, galtrap, calthrop, jackrock or crow's foot) is an area denial 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). Historically, caltrops were part of defences that served to slow the advance of troops, especially horses, chariots, and war elephants, and were particularly effective against the soft feet of camels. In modern times, caltrops are effective when used against wheeled vehicles with pneumatic tires.
The modern name "caltrop" is derived from the Latin calcitrapa (foot-trap), such as in the French usage chausse-trape. The synonymous Latin word tribulus provides part of the modern Latin name of a plant commonly called the caltrop, Tribulus terrestris, whose spiked seed case can injure feet and puncture tires. This plant can also be compared to Centaurea calcitrapa, which is also sometimes referred to as the "caltrop". A water plant with similarly-shaped spiked seeds is called the "water caltrop", Trapa natans.
Caltrops were known to the Romans as tribulus or sometimes as murex ferreus, the latter meaning "jagged iron" (literally "iron jagged thing").  They were also used in the Battle of Carrhae in 53 BC.
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.
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 caltrop, or water chestnut (genus Trapa), formed a natural type of makibashi called "tennenbishi". Both types of makibishi could penetrate the thin soles of shoes, such as the waraji sandals, which were commonly worn in feudal Japan.
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 as riprap 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
World War I
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".
World War II
Caltrops were used extensively and effectively 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?].
The Germans dropped crowsfeet. These were made from two segments of sheet metal welded together into a tetrapod with four barbed points and then painted in camouflage colours. They came in two sizes with a side length of either 3 or 2 1⁄2 inches (76 or 64 mm). They were dropped from aircraft in containers the same size as a 500 kg bomb and were dispersed by a small explosive charge. They were used to attack airfields and roads with a view to bursting tires.
Caltrops have been used at times during labour strikes and other disputes. Such devices were used by some to destroy the tires of management and replacement workers.
Caltrops, referred to as "jack rocks" in news articles, were used during the Caterpillar strike of the mid-1990s, puncturing tires on vehicles crossing the picket line in Peoria. Because of their small size and the difficulty proving their source, both the company and the United Auto Workers blamed each other. Collateral damage included a school bus and a walking mail carrier. In Illinois, the state legislature passed a law making the possession of such devices a misdemeanor.
A caltrop has 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 also 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.
The Savannah Association annually commemorate "Operation Savannah" a battle from 1975 to 1976 in the South African Border War also known as the Namibian War of Independence. The battle involved a covert intervention by the South African Defence Force's (SADF) during the interrelated Angolan War of Independence, subsequent Angolan Civil War and Namibian War of Independence. During this battle many South African soldiers earned the Honoris Crux for bravery against Angolan and Cuban forces. Their insignia was a caltrop.
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- Booby trap
- Czech hedgehog
- Knucklebones, a game, with similar hazards
- Spike strip
- Tribulus terrestris, a plant
- Water caltrop, a plant
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