Distinction and the Rule of Perfidy within the Electromagnetic Spectrum

Editors’ note: This post is based on course work that Luke Gigliotti completed in his final year as a cadet at the U.S. Military Academy West Point.

The electromagnetic spectrum (EMS) plays a crucial role in contemporary armed conflicts. The challenge for military units in operating without being detected by electromagnetic (EM) systems has stimulated efforts to conceal or disguise military electromagnetic emissions (EMEs), including by producing EMEs that are not distinctly military during conflict. Like any element of warfare, this concealment must be compatible with the applicable law of armed conflict (LOAC). This post examines how the LOAC principle of distinction, and the related rule of perfidy, may apply to military efforts to avoid detection by EM means.

The EMS

A caveat: this post is legal rather than technical. In it, we consider broad approaches to concealing military EMEs, some of which may not (yet) be technically feasible. It is worth sketching the basic nature of the EMS, however. The EMS can be defined as “a series of frequencies ranging from radio waves to microwaves, visible light, X-rays, and gamma rays.” All individuals and objects have an EM signature, whether because they radiate heat, contain functioning electrical circuitry, because they are designed to produce EMEs (e.g. radio waves for communication or active detection purposes), or simply because they can be seen. Military units necessarily produce EMEs, as do civilians and civilian objects. Thus, both military and non-military EMEs will be present in the EMS during a conflict. Modern EM detection technologies can passively identify, locate, and distinguish different EMEs, producing valuable military intelligence.

As a basic example, NATO’s Joint Civil/Military Frequency Agreement established separate frequencies for military and civilian use, to avoid conflicting broadcasts. An EME that matches those produced by a military radio, such as on a NATO military frequency, would thus strongly suggest the presence of a military unit at the emitter’s location. Using more complex techniques, the presence of particular types of vehicle can be passively detected via their unique EM signatures, sometimes at a rate of up to 99.3% accuracy.

For obvious reasons, LOAC rules relating to distinction in the visual parts of the EMS have existed for many years. In this post, we focus on concealment and disguise in those parts of the EMS that are non-visual. (In addition to visual light, we do not consider parts of the EMS that can be viewed through special equipment to produce what is essentially a visual image; e.g. infrared heat signatures, viewed through appropriate optics to give an image similar to that in the visual spectrum).

EM Detection in Conflict

The conflict in Ukraine demonstrates that high-quality EM detection can significantly enable combat operations. Ukraine has been able to locate and target Russian generals by geolocating emissions from their personal cellular devices with significant strategic effect. Russia initially struggled to use its full capability in a congested EMS, partially due to civilian emissions. The conflict also demonstrates the increased likelihood of States targeting enemy forces with “unobserved fires,” i.e. using artillery to bombard areas that are not visually monitored by the employing force. These can be controversial, as it is debatable whether they meet the requirement to take all feasible precautions to limit effects on protected persons and objects in attack (see for example Additional Protocol I to the Geneva Conventions (AP I), art. 57). The United States has not generally used such fires in the recent past. It has, however, taken the position (see p. IV-12) that fires can be considered “observed” if the targeted area is monitored by certain non-visual means, including EM detection, provided this otherwise meets legal requirements for targeting.

The Ukraine conflict has also demonstrated how brief the “sensor to shooter” process (between detecting an EM emitter’s location and then striking it with lethal precision weaponry) can be. It is therefore unsurprising that militaries seek to conceal or disguise their EMEs by investing in technology such as advanced camouflage nets that “manage” units’ signatures. In 2023, the U.S. Army Small Business Innovation Research Program released a contract opportunity to develop “Electromagnetic Skins and Smart RF Radomes for Spectrum Camouflage.” The desired skins are “a thin layer of radio frequency (RF) components and/or periodic structures conformed to an Army platform that manipulate radiation or scattering parameters,” i.e. physical components placed into or on an item of military equipment (such as a physical dome surrounding an antenna) to both limit the EMEs it produces and enable manipulation of its EMEs to mislead passive detection systems. Such a skin would thus not only allow the U.S. Army to blend into the environment, but also to deceive an enemy actively.

Distinction, Perfidy and Ruses of war

The International Court of Justice described distinction as a “cardinal” LOAC principle in the Nuclear Weapons advisory opinion (para. 257). It exists in two forms. The first is the responsibility of a party to an armed conflict to distinguish between lawful military targets and protected persons and objects, including civilians, in attack (see e.g. AP I, art. 48). This is sometimes known as “active distinction.” The second is the responsibility of a party to adequately distinguish their own forces from protected persons and objects, known as “passive distinction.” Together, these requirements safeguard civilians and other protected persons and objects.

Passive distinction is reflected in LOAC treaties and customary international law. For example, the first part of Article 44(3) of AP I (which of course does not bind certain States, including the United States) provides, “In order to promote the protection of the civilian population from the effects of hostilities, combatants are obliged to distinguish themselves from the civilian population while they are engaged in an attack or in a military operation preparatory to an attack.” Other rules arising from the principle include those requiring warships (§ 2.2.1; also see UN Convention on the Law of the Sea, art. 29) and military aircraft, likely including at least larger unmanned vehicles, to bear distinctive markings. There are also particular provisions for medical transports.

The application of the principle of distinction in the visual spectrum does not, of course, forbid militaries from camouflaging personnel or materiel to match their surroundings. Camouflage, the concealment of personnel, equipment, or structures, has been defined as a ruse of war, “to mislead an adversary or to induce him to act recklessly” (§§ 5.25.1, 5.25.2). LOAC thereby balances civilian protections (combatants must be distinguishable as such when seen) against the practicalities of warfare (combatants may camouflage themselves to avoid detection).

Perfidy is a prohibited activity and a war crime. Article 37 of AP I defines perfidy as “[a]cts inviting the confidence of an adversary to lead him to believe that he is entitled to, or is obliged to accord, protection under the rules of international law applicable in armed conflict, with intent to betray that confidence,” in order to kill, wound, or capture the adversary. Perfidy also appears in the Hague Regulations at Article 23(b); the U.S. Department of Defense Law of War Manual (DoD Manual) (§ 5.22); and the International Committee of the Red Cross’s Customary International Humanitarian Law study (rule 65). Perfidious attacks are likely to have more serious consequences than simple failures of passive distinction; the former explicitly amount to a war crime (see for example the Rome Statute of the International Criminal Court, Articles 8(2)(b)(xi) and 8(2)(e)(ix)), the latter (of themselves) are not.

The elements to perfidy can be separated as: (1) an act or acts inviting the adversary’s confidence done to make the adversary believe they are entitled or obliged to provide protection under LOAC; (2) intent to betray that confidence; and (3) intent thereby to kill, wound or capture the enemy. Professor Watts notes that perfidy stemmed from the idea of honourable combat, reflecting a minimum standard of respect and trust between opponents. The prohibition of perfidy upholds LOAC’s purpose of maintaining humanity during armed conflict, and reducing suffering.

There are some partial exceptions to the passive distinction requirement. For example, Article 44(3) of AP I allows “that there are situations in armed conflicts where, owing to the nature of the hostilities an armed combatant cannot so distinguish himself” and thus states combatants do not lose their status or commit perfidy if in such situations, they carry their arms openly:

a) during each military engagement, and

b) during such time as he is visible to the adversary while he is engaged in a military deployment preceding the launching of an attack in which he is to participate.

Furthermore, notwithstanding the general LOAC requirement to act in “good faith” (DoD Manual § 5.21), ruses of war, including use of feints, decoys and false information, are lawful. These are “acts that are intended to mislead an adversary or to induce him to act recklessly, but that do not infringe upon any rule of international law applicable in armed conflict and that are not perfidious because they do not invite the confidence of an adversary with respect to protection under that law” (DoD Manual, § 5.25.1). The acceptability of ruses was recorded within Article 24 of the Fourth Hague Convention of 1907 and history contains many examples of these. They are not perfidy, because they do not exploit an enemy’s fraudulently-generated confidence that persons or objects are protected by LOAC.

How May Military Units Lawfully Manage, Conceal, or Disguise Their EMEs?

We now consider to what extent military units must passively distinguish themselves in the EMS, and whether attempts to conceal or disguise a unit’s EMEs may breach the passive distinction requirement, or even amount to perfidy.

EME “Markings?”

The principle of distinction seems unlikely to require that military units produce distinct, and otherwise unnecessary, non-visual EMEs, in order to “mark” themselves as military. There is no explicit LOAC rule to require this. Furthermore, such units operate within a physical domain (the land, sea, or air). Combatants and vehicles must already visually distinguish themselves within these domains and thus may be distinguished by their opponents, if located, without the need for EME markings.

Requiring EME markings would also mean that the unit must actively advertise its location in the EMS. This seems to go well beyond the requirements currently imposed by passive distinction, which do not require a unit to flag its presence or even be visible, but only to ensure it is distinguishable if seen, so that it may be actively distinguished by the enemy. Only “feasible” passive precautions are required by AP I (see art. 58). Passive distinction requires military units to be distinguishable from civilian objects, not to operate a siren. The requirement might even compel units otherwise minimizing their EMEs to carry additional equipment to produce markings. A requirement to mark a unit’s location would thus upset LOAC’s balance between active and passive distinction and seems wholly unrealistic. EME markings can, however, actively identify genuinely protected persons and objects, thus preventing accidental targeting of these. Annex I of AP I establishes specific radio and electronic identification signals (an identification code) exclusively for the use of medical vehicles and aircraft (see AP I, annex I, ch. 3 , arts. 6, 8, 9).

Decoy Military EMEs

As with the inflatable decoy armies of the Second World War (for whom significant decoy radio traffic was generated), the use of decoy EM emitters that simulate the presence of a non-existent military unit will be a lawful ruse (§ 5.25.2), assuming these are used in accordance with LOAC (e.g. situated to minimize potential civilian harm). As with other ruses, such emitters do not suggest the presence of a protected person or object. They invite enemy confidence that a phantom military unit is present, not that any protection must be provided. Equally, concealing an actual military object’s EMEs as those of a different (unprotected) military object is a lawful ruse (§ 5.25.2.1) as it does not invite confidence in the presence of a protected object.

Concealing or Disguising Unit EMEs

Units that do not conceal or disguise their unavoidable EMEs from a sophisticated adversary may not survive. Concealing a unit’s EMEs so that they are completely undetectable, or appear to be naturally occurring EMS phenomena, will almost certainly be lawful. A clear analogy can be drawn between these and the use of natural camouflage to conceal a unit’s presence. In both cases the intended effect is that no object or only a naturally occurring object appears to be present, not a protected person or object that the military unit should be distinguished from.

While avoiding detection altogether may be desirable, it may not be technically or militarily feasible. The more difficult question is whether military units may lawfully produce EMEs with a signature that is not distinctly military. This may occur when equipment and networks normally used by civilians are employed for military purposes (e.g. military units communicating via encrypted cellular devices on civilian networks). Alternatively, technology may be used to manipulate EMEs so that they present as non-military. In both cases, the military unit’s EMEs will be less obvious within the EMS, thus making detection harder, often termed “hiding in the noise.”

It can be argued that EMEs that are not distinctly military must therefore be of “civilian” appearance, especially if the definition of civilian objects is analogously applied. Thus, it may be suggested that military units must emit distinctly military EMEs to comply with passive distinction requirements and to avoid the commission of perfidy. We are unconvinced by this argument, for the reasons below.

Producing non-military EMEs does not of itself reach the perfidy threshold. This is because (for the reasons below) producing such EMEs should not invite an enemy’s confidence that the emitter is civilian, so that the enemy wrongly believes it is protected by LOAC (for a similar argument relating to visual invisibility, see here, p. 316). Nor, without more, is there intent to engender or to betray such a confidence.

There is no LOAC rule explicitly requiring that military units produce distinctly military EMEs. Furthermore, producing non-military EMEs does not amount to a representation that the emitting object has no military purpose or use, or is otherwise protected. Militaries routinely use repurposed civilian objects and technology. Russian military use of personal cellular devices in Ukraine was considered foolish, but not unlawful. Both sides regularly use repurposed “off-the-shelf” civilian drones for reconnaissance and dropping munitions, and a variety of repurposed civilian—and enemy—vehicles for transport (some of which have been visually distinguished, e.g. with Russia’s “Z,” or flags). Armed groups can attain combatant status under LOAC without forswearing civilian equipment. The speed of technological advances means that equipment purchased “off the shelf” will be increasingly used by militaries of all sizes.

With so many military EMEs being generated via civilian technology (and thus being ”non-military” in signature) it is difficult to maintain that producing non-military EMEs is a positive representation by the emitting unit that it is civilian or otherwise protected. All parties to a conflict will be aware that enemy units may produce EMEs that are not distinctly military and that these do not represent an object protected by LOAC. Contrast, however, instances such as a military aircraft deliberately “squawking” an identification to designate itself as a civilian airliner; this involves the use of a positive signal with the specific meaning that the emitter is a protected object.

A military object producing non-military EMEs will not itself be disguised as a protected object, it must still carry markings prescribed by LOAC, and may still be distinguished by visual means. While its EMEs may not be obviously military to passive detection systems, intent to fool a machine does not constitute intent to create the level of misplaced confidence required for perfidy. This must be human (not machine) confidence that the relevant object is protected, grounded in all the circumstances. A tank producing an EME suggestive of a pickup truck may fool a detection system but is not positively holding itself out as civilian in its purpose and use (trucks may be put to military use), and employs no visual disguise. It is not analogous with, say, a tank disguised as a civilian vehicle or battlefield ambulance, which is deliberately held out as a protected object. While the drafters of AP I may only have contemplated visual disguises, and thus a distinction requirement in the EME is arguable, the former tank is producing an EME of an indistinct nature to avoid detection or make an adversary uncertain; the latter has adopted the appearance of a civilian vehicle to exploit its protected status.

Is producing non-military EMEs a breach of the wider duty of passive distinction that combatants must distinguish themselves from the civilian population while engaged in or preparing to attack? This requirement is explicitly restricted to visual distinction in the DoD Manual (see § 2.5.3), but not in AP I. On balance, and without more, we submit this duty is not breached. The arguments made above against EME disguises amounting to perfidy also apply here. Additionally, LOAC permits military units to camouflage themselves within civilian environments, including those consisting almost entirely of civilian objects, such as the urban environment. Professor Nasu and Professor Schmitt have noted that conducting urban warfare in civilian areas does not of itself breach LOAC. Where militarily necessary, and subject to rules on minimising civilian harm, civilian infrastructure can be used for cover or concealment. Combatants and military objects may be covered in camouflage patterns (including urban and “multicam” patterns) and use items (a sniper’s “ghillie suit,” or camouflage netting placed over a tank, for example) to alter their appearance to blend into civilian infrastructure. Producing EMEs that better blend into the wider, mostly artificial environment of the EMS is arguably no different.

LOAC’s partial allowance for camouflage and concealment amongst civilian objects demonstrates that the passive duty of distinction is not absolute, but qualified by feasibility, and balanced with the attacker’s duty of active distinction. The duty of passive distinction does not require absolute separation of the military and the civilian, or compel military units to make themselves painfully obvious to an enemy already required to distinguish military targets actively. Mandating that all military units sport a particular shade of orange or that warfare only take place in open fields would make distinction far easier, but would upset LOAC’s careful balance between humanity and military necessity, and be unrealistic. Equally, the balance dictates that combatants may alter their EMEs to avoid easy detection in the EMS. While combatants cannot plead basic force protection to escape the requirement to distinguish themselves, the principle of distinction has never been sensibly interpreted to require suicidal practices in the first place.

For similar reasons, concealing military EMEs so that their nature is simply unclear to the adversary, perhaps by using a skin or constantly changing signals, is also likely to be lawful. This does not invite the necessary level of confidence for perfidy, and, again, military units are not required to produce distinctly military EMEs. We do not consider the requirements for mimicking enemy EMEs here.

Producing EMEs to Resemble a Protected Object

The situation might be different if a military unit were to produce EMEs that positively assert the presence of a protected object (such as the military aircraft mentioned above, if it emits a distinct, positive signal for a civilian airliner or medical transport to avoid being intercepted as it conducts an attack), or if non-military EMEs are emitted with perfidious intent, possibly alongside visual indicators of protected status (e.g. a military unit in civilian clothes, intentionally using only unmarked civilian trucks and producing only non-military EMEs, at the commencement of an attack). If done with an intent to betray the relevant confidence in order to kill, wound, or capture, these seem likely to meet the perfidy threshold. Positively asserting the presence of civilians to render a unit or area immune from attack may also breach Article 51(7) of AP1, and potentially Article 58 of AP I where the relevant side has civilians under its control.

Conclusion

LOAC has always struck a careful balance between humanity and military necessity and has limited the steps that military units must take to distinguish themselves accordingly. Allowing concealment of military EMEs, and production of EMEs that blend into the wider artificial EMS environment, seems in keeping with this balance. Any insistence that military units must emit only distinctly military EMEs, and thus hasten their detection by contemporary systems, makes little allowance for modern military necessity. However, a military unit deliberately producing EMEs that amount to a positive representation of the emitter’s civilian nature, to unlawfully hide behind LOAC’s protections, seems likely to breach the law.

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Photo credit: armyinform.com.ua