Although it was used for centuries by numerous armies, the unobtrusive-looking device called the caltrop, or calthrop, has often been overlooked by military historians, but certainly not by anyone unfortunate enough to have encountered it under field conditions. In many respects, the caltrop is the ideal passive weapon–simply constructed, cheap and easy to manufacture, requiring no special skill or training to use, easily portable, needing no care, maintenance or preparation, capable of recovery and, above all, extremely effective in most settings. It has killed or disabled innumerable soldiers, horses, camels, elephants and even land vehicles equipped with pneumatic tires. Silent, insidious and decidedly not glorious, the caltrop has few admirers. On the other hand, it has never been denounced in the same way as have the crossbow, poison gas, land mines and a whole arsenal of other weapons, ancient and modern. And unlike other weapons, it has never been completely replaced by more modern descendants.
The original caltrop was nothing more than a ball from which four spikes projected in such a way that when three spikes were on the ground the fourth was always pointed upward. To step on it was to risk a laceration or puncture wound–painful, debilitating and hard to heal–which could result in serious infection or a slow death. The caltrop, therefore, bears a close family resemblance to snares, stakes, trenches and pits used singly or in combination to entangle or injure the feet of men and animals. Like those devices, it might well have originated as a hunting trap.
The word ‘caltrop’ originally meant the star thistle, a weed whose form and function were similar to those of the weapon. The Greeks called the device a tetrahedron or tribolos, again because of its shape. Borrowing the Greek word, as they so often did, the Romans referred to it as a tribulus, but they also called it a murex, because of its resemblance to the shell of the murex, the mollusk from which the Tyrians obtained their famous purple dyes. Under a wide variety of names, the caltrop appears and reappears in military history throughout Europe, Asia and North Africa, and eventually in the New World.
As is the case with so many premodern weapons, the caltrop’s origins remain shrouded in mystery. It is possible that its earliest recorded use occurred in Persia, at the Battle of Arbela (Gaugamela) on October 1, 331 bc. At any rate, Polyaenus of Macedonia, writing 500 years after the fact, claimed that Persian King Darius III sowed some of the ground in front of his army with ‘crow’s-feet’ in order to restrict the enemy’s freedom of movement. Alexander the Great, however, was able to maneuver around those devices, penetrate the Persian line of battle and win the day. If the ‘crow’s-feet’ were actually caltrops, they must already have been familiar for some time, for Polyaenus does not describe them further or state that they were a new invention. And although they may not have foiled Alex-ander at Gaugamela, he and his successors seem to have considered them a weapon worth using.
After Alexander’s death, his self-appointed successors struggled for supremacy and territory within the vast empire they had helped to conquer. In the ensuing wars between the various Hellenistic monarchies, extensive use was made of wooden balls armed with metal spikes, which formed a valuable component in field and camp defenses and in the perimeter protection of fixed fortifications. Sown on the battlefield, and sometimes partially buried, they were much more difficult to detect than elaborate, time-consuming systems of pits and stakes and served to discourage attacks on vulnerable sectors of the line. Of course, a thick belt of caltrops also restricted the movements of one’s own troops, but that was a small price to pay for the security they gave. Although not immediately lethal, the devices caused wounds that sapped an enemy’s morale. The sight of injuries inflicted on horses or comrades by caltrops made infantry and, even more, cavalry uneasy about advancing over ground that might be strewn with the insidious devices–thus restricting their offensive capabilities to a greater extent, perhaps, than actual combat fatalities. Moreover, an adequate supply of caltrops was an excellent guarantee against the danger of a night attack.
The Romans, with their usual flexibility in matters of weaponry, were quick to adopt the caltrop from the various Hellenistic armies and military engineers they encountered. Surprisingly, Gaius Julius Caesar made no use of caltrops in the great siege works he erected around the Gallic stronghold of Alesia in 52 bc, but his ‘blocks of wood…with iron hooks fixed in them,’ called ‘goads’ by his legionaries, served the same purpose.
At the Battle of Nisibis (modern Nusaybin, Turkey), fought in the summer or early autumn of 217 ad, the Romans gave a brilliant demonstration of the proper use of caltrops in combat conditions. Artabanus V, the last Parthian Shahanshah (king of kings), was justly incensed by a treacherous massacre–from which he himself had narrowly escaped–that had been engineered by the Roman Emperor Bassianus Marcus Aurelius Antonius (aka Caracalla) during negotiations for an alliance. The notorious Caracalla was murdered by one of his own officers in April 217 and replaced by Marcus Opelius Macrinus, but the Parthian ruler was not to be appeased. Both Artabanus and Macrinus took to the field, commanding their respective armies in person. Artabanus was already a veteran commander, while the new Roman emperor, although he had been praetorian prefect before his ‘election’ by the army, lacked practical military experience.
There might have been some preliminary skirmishing over access to water, but the Parthians seem to have achieved almost complete tactical surprise. Artabanus immediately took the initiative and, shortly after sunrise, opened the battle with a furious charge. While his archers laid down a hail of arrows, Artabanus’ heavy cavalry advanced at a gallop, accompanied by armored lancers mounted on camels. The Moorish soldiers and cavalry on the Roman wings, and the light-armed troops in the center, withstood the assault and fought back bravely, but the sheer weight of the Parthian attack soon overwhelmed them.
Macrinus must have had excellent advice from his staff, as well as highly disciplined and experienced troops, because at that point the Romans pretended to retreat, while throwing down caltrops and other iron devices with spikes sticking out of them. The caltrops sank into the sand, and the Parthians failed to see them in time. Horses and camels were made lame and brought down and their riders thrown to the ground. The Romans, too, had taken heavy casualties, but they had definitely foiled the Parthian charge. The battle lasted for two more days and ended in something like a draw, although Macrinus found it prudent to make reparations to Artabanus. The Roman emperor had had a close brush with disaster, and he was overthrown in the following year by Caracalla’s cousin, Varius Avitus, while Artabanus fell victim to a revival of Persian power under Ardashir of Sassan in 226.
The next notable example of the use of caltrops occurred in December 637 at the Battle of Jalula, during the conquest of the Sasanid Persian empire by the Muslim Arabs. In June of that year, Ctesiphon, the winter capital of Yazdgard III, the last Sasanid Shahanshah, had fallen to the Arabs after the Persian ruler fled. Ctesiphon lay on the Tigris River, about 40 miles below modern Baghdad, and the Persians were determined to recover the city.
By autumn, Yazdgard once again had considerable forces at his disposal, and he sent one of his remaining generals, Mihran, forward to the fortress town of Jalula, located on the fringe of the Iranian highlands, some 90 miles northeast of Ctesiphon. There the Persian forces constructed a large fortified camp, surrounded by stakes and extensive fields of caltrops.
Sa’d ibn abi-Waqqas, the Arab commander, sent two of his generals and 12,000 men to attack the much larger Persian army. According to some accounts, there was some indecisive fighting, but the Arabs were reduced to blockading the strong Persian position for the next 80 days. The blockade was not particularly effective, for both sides received supplies and reinforcements. Finally, Mihran, possibly deceived by a feigned Arab withdrawal, confidently ordered his troops to sweep lanes through their own caltrops and advance on the enemy. Unfortunately for him, the Arabs held the attack, then forced the Persians back, so that many of them fell victim to their own caltrops. The result was catastrophic. During the battle and the pursuit that followed, Persian losses were so heavy (100,000 men, according to inflated Arab claims) that Yazdgard abandoned Ctesiphon to its fate and fled on to Ray, outside mod-ern Tehran.
In the Middle Ages, European smiths improved and simplified the caltrop design by eliminating the ball, twisting two double-pointed strips of iron and cold-hammering them together. The caltrop now resembled, more than ever, the ground thistle from which it took its English name. By that time, too, the caltrop was in use throughout Europe, Asia and North Africa. In fact, medieval China witnessed what was probably the largest single deployment of the device in military history.
In late July or early August 1213, Genghis Khan brought his armies back into North China in order to renew the Mongol assault on the Chin empire of the ‘Golden Tatars,’ a dynasty of Manchurian origin. The northwest approach to Chung Tu, the main Chin capital, was guarded by a huge fortress complex in the Chü-yung Kuan pass. Although seemingly impregnable, the Chü-yung Kuan had actually fallen to the famous Mongol general, Jebe Noyan, just over a year before. Pressed for time, and realizing that his detachment was not strong enough to storm the Chin fortifications, Jebe had lured the garrison out by a feigned retreat, then virtually annihilated the Chin troops 35 miles from their base. The Mongols soon abandoned their prize, however, since they had little use for permanent fortifications, concentrating instead on depleting the much more numerous military manpower of the Chin.
Meanwhile, the Chin authorities had not been idle. They strengthened the main Chü-yung Kuan fortress with trenches and other works, and they took special precautions with the P’ei K’ou fort at the north entrance to the vital pass. According to some records, the gates of the forts were sealed with iron and the surrounding country for 100 li (approximately 30 miles) strewn with caltrops. Genghis Khan himself waited in front of the fort for more than a month before leaving to deal with less formidable obstacles. The Mongols must have compelled vast numbers of Chinese civilians to clear away the belt of caltrops, for they blockaded the P’ei K’ou so effectively that its food supplies gave out and the garrison was even reduced to cannibalism. In any case, the Mongol holding force, under Kita and Bukha, did not have long to wait for success. Jebe Noyan and an equally re-nowned general, Sübotei, swept through the Chü-yung Kuan from the south, and the commandant of P’ei K’ou surrendered with his starving troops.
As time wore on, the caltrop declined in popularity in Europe, for reasons that are not altogether clear, although the growing use and refinement of gunpowder weapons must have played a part in its eclipse. Caltrops were still used for defense in sieges, where they could be thrown into breaches in fortifications to impede storming parties. Leonardo da Vinci even designed machines for hurling baskets of caltrops in the direction of the enemy. Only the Swedes continued to use caltrops in any quantity in the field, doing so as late as the 18th century. Ironically, just as the caltrop was falling out of favor in Europe, it found a short-lived application in the New World. The first English settlers at Jamestown, Va., brought with them a supply of caltrops, ideal for discouraging surprise attacks by the American Indians should relations turn sour.
Most caltrops, like those from Jamestown, are quite small, and those forged by the Japanese had spikes only about an inch long. Some Indian examples, on the other hand, designed for use against elephants, were relatively large and elaborate, resembling Caesar’s ‘goads’ more than caltrops. Whatever its size, however, the caltrop is dangerous to man and beast. In fact, it is so potent an agent of infection–being exposed to contamination by soil and weather–that attempts to deliberately apply poison to it seemed unnecessary.
Despite its shifting fortunes, the caltrop remains very much with us. Its use was revived during the Korean War, when it was employed effectively against sneaker-shod Chinese infantrymen by United Nations forces. Today, it has reclaimed its old Greek name and reappeared as the tetrahedron, the bane of all vehicles running on pneumatic tires, and is used by both the military and police. Beside this versatile, durable and diabolical little device, its alleged descendant, the barbed-wire entanglement, seems quite prosaic.
This article was written by Robert W. Reid and originally appeared in the August 1998 issue of Military History magazine. For more great articles be sure to subscribe to Military History magazine today!