Caltrop: An Ancient Weapon in Modern Warfare


| May 20, 2024


Recent reports indicate that Ukraine has modified drones to deliver and disperse a weapon of ancient design in its fight against Russia. The weapon, known as a caltrop, is a spiked device that can be strewn in large numbers, sometimes connected by a chain, across a roadway or other terrain to impede an enemy’s movement in an operational area. Modifying drones to deploy caltrops is not a new idea, but Ukraine’s use of drones in this way represents the first significant example in armed conflict. This post explores some legal issues associated with drone deployment of caltrops in armed conflict.

What Is a Caltrop?

A caltrop is an ancient device similar in appearance to a pronged jack in the children’s game of jacks. Rather than prongs, caltrops feature pointed spikes intended to hinder the movement of enemy soldiers and vehicles. A common design features four spikes positioned to allow three to rest on the ground while one remains pointed upward. Caltrops can also take the form of a metal ball covered in spikes. Because caltrops are simple to manufacture, inexpensive to produce, and easy to deploy, they can make an effective tool for area denial.Caltrop

Caltrops have been employed for centuries against both infantry and mounted troops. The Romans are said to have neutralized enemy armed chariots by scattering caltrops across the battlefield. During the Middle Ages, caltrops remained in common use and were sometimes represented as heraldic devices on coats of arms. In modern times, caltrops have also been used to inhibit vehicles with pneumatic tires. Both the Axis and Allied powers used caltrops during the Second World War, and a German version, known as the “crowsfoot,” could be dispersed by air over roadways and airfields. Symbolic reminders of the caltrop’s significance are still evident in the U.S. armed forces today. The insignia of the U.S. Army’s III Armored Corps, for example, is a blue caltrop, and the 3rd Marine Division’s unit insignia also incorporates a caltrop.

Non-Lethal Weapons

Caltrops can be characterized as a non-lethal weapon, although it is important to note that “non-lethal weapons” are not a defined class or category of weapons under the law of armed conflict. The U.S. Department of Defense (DoD) Law of War Manual describes non-lethal weapons as “weapons, devices, and munitions that are explicitly designed and primarily employed to incapacitate targeted personnel or materiel immediately, while minimizing fatalities, permanent injury to personnel, and undesired damage to property in the target area or environment” (§ NATO’s definition similarly emphasizes their design to “incapacitate or repel personnel, with a low probability of fatality or permanent injury, or to disable equipment, with minimal undesired damage or impact on the environment.”

United States policy further explains that “[u]nlike conventional lethal weapons that destroy their targets principally through blast, penetration, and fragmentation, [non-lethal weapons] employ means other than gross physical destruction to prevent the target from functioning” (DoD Dir. 3000.03E, para. 3(c)). Despite being explicitly designed to incapacitate or repel rather than kill, non-lethal weapons can nevertheless cause fatalities.

The DoD Law of War Manual acknowledges that non-lethal weapons “do not have a zero probability of producing fatalities or permanent injuries” and that their use “may still present risks of serious injury or loss of life” (§ William Boothby comments that “[i]t is therefore immediately clear that the term ‘non-lethal’ is an inappropriate description” (p. 234). “Less-lethal” is sometimes used in place of “non-lethal” to describe these weapons. Examples of non-lethal or less-lethal weapons include riot control agents, blunt impact projectiles, dazzling lasers, some directed energy weapons, electrical stun guns, and counter-material weapons that are designed to stop vehicles without harming the vehicles’ occupants (see U.S. DoD, Law of War Manual, §

As already noted, non-lethal weapons are not recognized as a distinct class of weapons under the law of armed conflict. Accordingly, they are regulated by general customary rules applicable to weapons and to specific rules, such as the prohibition against the use of riot control agents as a method of warfare, as applicable (§ Among these rules is the requirement to conduct legal reviews to ensure new means and methods of warfare comply with international weapons law. Whether these legal reviews are required as a matter of customary international law is subject to some debate. But for States party to Additional Protocol I (AP I), like Ukraine, compliance with the legal review requirements specified in Article 36 is obligatory.

One legal issue drone-deployed caltrops raise is whether dispersal of caltrops by drone is a lawful method of warfare. Drones have been employed in innovative ways throughout the Ukraine conflict, including as modified grenade delivery systems and as facilitators of surrender. In these instances, they have proven remarkably capable of precision guidance. Because drones can be directed at combatants and military objectives, drone-deployed caltrops are unlikely to be considered inherently indiscriminate, and such use is unlikely to violate international weapons law prohibitions.

Distinguished from Booby-Traps

A caltrop is not a booby-trap despite generally injuring or damaging by surprise. Amended Protocol II of the Convention on Certain Conventional Weapons defines a booby-trap as “any device or material which is designed, constructed or adapted to kill or injure, and which functions unexpectedly when a person disturbs or approaches an apparently harmless object or performs an apparently safe act” (art. 2(4)). Amended Protocol II further provides, “It is prohibited to use booby-traps or other devices in the form of apparently harmless portable objects which are specifically designed and constructed to contain explosive material” (art. 7(2)).

The DoD Law of War Manual suggests that the prohibition applies to items manufactured to resemble harmless objects, such as watches, personal audio players, cameras, and toys (§ The Manual explains, “The prohibition is intended to prevent the production of large quantities of dangerous objects that can be scattered around and are likely to be attractive to civilians, especially children” (§

A caltrop is clearly not designed and manufactured to resemble a harmless portable object. Although they can be small (approximately 3 to 5 inches in length), caltrops look intimidating and aggressive. Caltrops are also not intended to contain explosive material, and they do not activate merely when disturbed or approached. Rather, they are passive instruments that cause harm by puncturing objects with which they come into physical contact when enough force is applied. The concerns evident in Amended Protocol II’s prohibition, therefore, are not implicated when caltrops are employed as designed.

Precautions and the Obligation to Clear

Once released, caltrops persist in the environment indefinitely. Unlike landmines, another area denial weapon, they cannot be equipped to self-destruct or self-neutralize, and they are not designed to be self-deactivating. Accordingly, they can remain a hazard, including to the civilian population, until cleared. What, then, is a State’s obligation to clear caltrops once they’ve been dispersed? The answer to this question could depend on whether and when the use of caltrops constitutes an “attack” under the law of armed conflict and whether clearance is a feasible precaution under the circumstances.

With respect to devices such as mines and booby-traps, the DoD Law of War Manual states, “The use of mines, booby traps, and other devices is subject to the same rules and principles that govern the use of other weapons to conduct attacks” (§ 6.12.5). Article 49(1) of AP I states that “attacks” are “acts of violence against the adversary, whether in offense or in defence.” This definition is generally accepted as customary.

The targeting regime of the law of armed conflict regulates attacks in armed conflict including obligations to distinguish lawful targets, to apply the rule of proportionality, and to take feasible precautions in attack. Determining when the use of a mine constitutes an “attack,” however, is tricky, a point the International Committee of the Red Cross (ICRC) Commentary to AP I acknowledges. The Commentary asks, does the use of mines constitute an attack “when the mine is laid, when it is armed, when a person is endangered by it, or when it finally explodes?” (para. 1960). Caltrops, although not explosive by design, raise similar questions: When does their use qualify as an attack?

The ICRC Commentary suggests that the use of mines constitutes an attack when a mine directly endangers a person (para. 1960). With regard to caltrops, this standard suggests that the use of caltrops would constitute an attack when they become a direct danger to persons, potentially including civilians who may be traversing an area where caltrops have been released.

The rule of distinction explicitly prohibits targeting the civilian population (AP I, art. 51(2)) and indiscriminate attacks (AP I, art. 51(4)(b)). Accordingly, caltrops directed against enemy combatants and materiel, such as vehicles, will not violate the obligation to distinguish combatants from non-combatants and military objectives from civilian objects. However, even attacks that comply with distinction can result in civilian harm, and foreseeable civilian harm is a critical legal consideration in the targeting analysis.

The rule of proportionality applicable to attack provides that “[c]ombatants must refrain from attacks in which the expected loss of civilian life, injury to civilians, and damage to civilian objects incidental to the attack would be excessive in relation to the concrete and direct military advantage expected to be gained” (AP I, art. 57(2)(b); DoD, Law of War Manual, § 5.12). Accordingly, an employment of caltrops anticipated to result in civilian harm would not necessarily violate the law of armed conflict so long as the expected harm to civilians and civilian objects is not excessive in relation to the military advantage anticipated.

Lastly, combatants are obligated to take “feasible precautions in planning and conducting attacks to reduce the risk of harm to civilians and other persons and objects protected from being made the object of attack” (AP I, art. 57; DoD, Law of War Manual, § 5.11). The obligation to take feasible precautions is open to interpretation, and expectations of what is required may differ depending on whether a State applies feasible precautions as a matter of customary international law or as stated in Article 57 of AP I. Additionally, what precautions are considered feasible will be context-dependent.

One potential precautionary measure involves the cancellation or suspension of an attack based on new information regarding expected civilian casualties (DoD, Law of War Manual, § 5.11.4). Accordingly, when new information indicates that civilians are expected to enter an area where caltrops have been distributed, clearing the caltrops might constitute a feasible precaution. Clearance could be considered a form of cancellation or suspension of an attack intended to mitigate the risk of harm to civilians and civilian objects.

Concluding Thoughts

Caltrops are modest devices that have been used to injure, repel, and deter opponents in various contexts throughout history. They have been used to impede the movement of enemy forces on the battlefield and to stymie boarding parties at sea. They have been used by law enforcement to thwart criminal activity. Even James Bond, whose Aston Martin DB5 was elegantly fitted with a caltrop dispenser, relied on them.

Although caltrops are described as non-lethal or less-lethal weapons, they still qualify as weapons under the law of armed conflict. As such, they must comply with international weapons law. From a weapons law perspective, equipping drones to deploy caltrops in armed conflict does not appear legally problematic as drones can disperse caltrops in a way that is not indiscriminate.

Additionally, because scattering caltrops could result in attacks, their use must comply with the targeting rules of the law of armed conflict. They must not be directed at civilians or civilian objects, and they must not be employed, even against enemy combatants and military objectives, if their use would be expected to result in excessive incidental harm to civilians or civilian objects.

Lastly, combatants must take feasible precautions in planning and conducting attacks to protect civilians. In some cases, this might involve clearing caltrops that present a danger of causing civilian casualties that are excessive in relation to the military advantage expected to be gained.


LTC Ronald Alcala is an Associate Professor and Associate Dean for Strategy and Initiatives at the United States Military Academy at West Point. He is also Senior Military Fellow at the Lieber Institute for Law and Warfare.






Photo credit: and Central Intelligence Agency