A caltrop (also known as Caltrap, galtrop, cheval Trap, galthrap, galtrap, calthrop, jack rock, star nail, crow's foot, partisan nails, or in Latin: Tribulus or in Japanese: makibishi or Tetsubishi.) 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). They may be thought of as the landmines of antiquity useful to shape the battlefield and force the enemy into certain paths and approaches, or to provide a passive defense as part of a defensive works system. Caltrops serve to slow down the advance of horses, war elephants, and human troops. It was 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 derivation of the modern name "caltrop" is uncertain, but the Latin Tribulus is clearly derivative of the plant sharing similar hazards to sandaled or bare feet the Tribulus terrestris (Zygophyllaceae), whose spiked seed case can also injure feet and puncture tires. It can also be compared to the Star thistle, Centaurea calcitrapa, whose Latin name calcitrapa means "foot trap".
Iron caltrops were used as early as 331 BC at Gaugamela according to Quintus Curtius (IV.13.36). They were known to the Romans as tribulus or sometimes as Murex ferreus, the latter meaning 'jagged iron'.
This device was used with great success by the Scots against the English at the Battle of Bannockburn in 1314, to disable horsemen. The Drummond clan deployed welded nail caltrops, halting English cavalry in its tracks and saving vital Scots infantry in a battle that meant over 4,000 English troops lost their lives and Edward II had to retreat without shield and sword. Their use undoubtedly contributed to the resounding Scottish victory.
Punji sticks and caltrops were used in the Vietnam War, sometimes with poison or manure on the points.
In Britain, 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. Very much more common were concrete devices called dragon's teeth that were designed to wedge into tank treads. However, dragon's teeth are immobile, so the analogy with the caltrop is inexact. Another caltrop-like WWII defence is the massive, steel, freestanding Czech hedgehogs that were designed as anti-tank obstacles and were also used to damage ships and landing craft.
There have been a number of attempts to develop a caltrop like device that will deflate tires in a manner useful to law enforcement agencies or the military.
See also Spike strip
Caltrops were collected by Australian Light Horse troops 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". Examples from 1917 are kept in the Cart Museum in Bundeena, situated in the state of Victoria, Australia.
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," and have become an icon in pro-union rallies, often depicted on t-shirts, hats, or even worn as jewellry in some cases.
Because of the prevalence of caltrops during the Caterpillar strike of the mid-1990s, the state of Illinois passed a law making the possession of such devices a misdemeanor.
In the 1970s, activists in the United States deployed caltrops against the tires of logging trucks. Earth First! quickly condemned the practice, seeing it as a hazard to humans and animals.
A caltrop (also given as caltrap, galtrap or chevaltrap) has had a variety of symbolic uses and is commonly found in heraldry.
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 an area denial weapon. Fort Hood is the only installation in the US Army that has declared the caltrop to be a weapon prohibited in the barracks.