Invisible Soldiers

by | Mar 11, 2024


Images of drone strikes, hypersonic missiles, and directed energy weapons have cultivated imagination of how war might be fought in the future. Meanwhile, on-board cameras and satellites have made modern battlefields more transparent and accessible to the public. Although technological change has been a constant in warfare, the broader world is now able to witness the impact of advances in technology on the battlefield in stunningly clear and current relief.

Still, the impact of some technologically advanced means of warfare, such as cyber operations and electromagnetic interference, is less visible. Battlefield innovation is even reaching a stage where some objects can be rendered invisible across different light spectrums. For example, new military clothing designed to hide Ukrainian soldiers from Russian thermal imaging optics and cameras marks the latest such technological innovation in a quest for an “invisibility cloak.”

There is a range of methods to reduce visibility and probability of detection by enemy forces, such as traditional camouflage by blending into the surrounding environment and stealth technology that minimizes observability against radar, infrared, or other probe beams. Scientists are also taking advantage of modern technological innovation in material science and technology to explore new ways of concealing objects entirely from visual, infrared, and thermal ranges by bending light waves around them.

As we edge closer to the zenith of cloaking, military planners and commanders must remind themselves of the legal limits imposed on the use of invisibility technology as a method of warfare. This post, partly drawing on the author’s previous work (co-authored with Sephora Sultana), revisits the difference between legitimate ruse and prohibited acts of perfidy as the basis for identifying possible unlawful uses of invisibility technology. It also explores legal implications for human shielding and passive precautions when invisibility technology is used within a civilian populated area.

Ruses of War

Deception is a basic method of warfare. The use of deception is abundant in military history, including: the Trojan Horse made by the Greek in an effort to break the defense of the city of Troy in the 12th Century B.C.; Nathan Bedford Forrest’s message of a “secret weapon” to induce a Union commander into surrender without a fight during the American Civil War; and dummy warships created by adding ballast and funnels to merchant vessels during the First World War. As Sun Tzu observed, “All warfare is based on deception” (para. 18).

Camouflage is a basic and prevalent form of deception to conceal friendly forces’ locations, capabilities, or intent. The U.S. Army’s Field Manual 3-90 on tactics identifies five fundamental methods of camouflage: (1) hiding by concealing an object with some form of physical screen; (2) blending materials into the background; (3) disguising to mislead the enemy; (4) disrupting to alter or eliminate regular patterns and target characteristics; and (5) decoying by deploying a false or simulated target (para. 8-113). Cloaking is similar to camouflage in many respects. It is a type of optical deception making a person or an object invisible across the visible light spectrum and undetectable against any background environment. As such, it can be considered as a technologically enhanced method of hiding for camouflage.

In principle, camouflage is a lawful method of warfare, generally known as a “ruse” under the law of armed conflict. Ruses in war seek to “mislead an adversary or to induce [them] to act recklessly” (§ 5.25.1) without infringing upon any rule of international law applicable in armed conflict. Cloaking soldiers or military assets is not unlawful per se if it is designed to avoid enemy detection and induce the adversary to make an error or to act imprudently as a result.

Nonetheless, lawful uses of cloaking may entail other legal effects on invisible soldiers if captured. Cloaked soldiers may well be denied prisoner-of-war status on the grounds that they were acting clandestinely for intelligence gathering behind enemy lines with the intention of communicating it to friendly forces (1907 Hague Regulations, art. 29). Or their prisoner-of-war status might be denied for committing an act of sabotage by damaging or destroying enemy assets (Department of Defense Law of War Manual, § 4.17).

Traditionally, this denial of a prisoner-of-war status has applied to unprivileged belligerents who deliberately concealed or mispresented their identity and conduct by wearing civilian clothes or an enemy uniform. The application of this exclusion rule to soldiers who are in fact wearing their own uniform while being cloaked is uncharted territory. Detaining authorities may or may not decide to treat cloaked soldiers as unprivileged belligerents due to the secret nature of the hostile activities they conducted. However, that itself does not render covert activities unlawful under the law of armed conflict. It simply means that captured individuals could be denied prisoner-of-war status and associated protection.


Invisibility technology must not be used to commit perfidy. Killing or injuring an adversary by resort to perfidy has long been recognized as an unlawful method of warfare (for example, 1907 Hague Regulations, art. 23(b)). Article 37(1) of Additional Protocol I to the 1949 Geneva Conventions 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 the intent to betray that confidence.” This strict definition is designed to prohibit only a specific category of deceptive act without otherwise interfering with the belligerent’s freedom to employ deceptive tactics that have traditionally been permitted as ruses of war. Thus, the Lieber Code provides, “[m]ilitary necessity . . . admits of deception, but disclaims acts of perfidy” (art. 16).

The use of invisibility technology merely for protective purposes may not amount to perfidy. It is therefore wrong to assume that invisibility technology is by its nature perfidious. Rather, the legality of invisibility technology depends on the specific manner in which it is used as an aid to military operations. It is not perfidious to cloak soldiers or military assets unless: (1) that action invites an adversary to believe that they enjoy or owe protection under the law of armed conflict; (2) with an intent to betray the adversary’s confidence in the law protection; (3) with the proximate result of killing or injuring a person.

An invisible soldier’s action could qualify as such when cloaking is used to disguise the status of personnel or an object as a means of misleading the adversary that they are entitled to protection under the law of armed conflict for the purpose of killing or injuring them. To qualify as perfidy, invisibility technology would have to be used to disguise a soldier’s status or conduct as civilian or hors de combat, or to conceal a soldier under the distinctive emblem or other protective signs recognized by the Geneva Conventions. The law of armed conflict does not accord any protection for those who are simply invisible or undetectable.

For example, invisible soldiers hiding in a civilian vehicle through military checkpoints with a view to breaching the enemy’s forward defense would qualify as perfidy. So would an armed hoverboard vehicle equipped with invisible weapons disguising itself as a commercial air delivery drone as a means of assassination.

The assessment becomes more difficult when the invisible soldiers’ intent to betray the adversary’s confidence in the legal protection is detached from the act of inviting that confidence. Consider, for example, invisible soldiers launching an attack against enemy soldiers when the latter have disarmed themselves in order to render assistance to captured wounded soldiers, who belong to the same armed forces as the invisible soldiers. In this case, the presence of the wounded soldiers invites the enemy soldiers to believe that they owe a duty of protection under the law of armed conflict. Whether the invisible soldiers intended to betray that confidence and thereby engaged in perfidy depends on the level of communication, planning, and coordination between the wounded soldiers and the invisible soldiers’ unit.

Committing an act of perfidy is not necessarily unlawful, let alone a war crime. Its legal consequence differs depending on the applicable law and how perfidy is committed, as my colleague Sean Watts has fully elaborated elsewhere. Invisible soldiers who hide themselves in a civilian vehicle to carry out an intelligence gathering or rescue mission without causing an adversary’s death or injury do not fall afoul of the perfidy prohibition. If they capture an enemy soldier by doing so, that would be unlawful only under Article 37 of Additional Protocol I. But such acts do not constitute a grave breach of Additional Protocol I unless the act of perfidy involves the use of protected emblems (Additional Protocol I, art. 85(3)(f)).

Human Shielding

Even in the absence of perfidy, the use of invisibility technology may still raise issues in relation to the principle of distinction. During an armed conflict, there is an obligation to distinguish at all times combatants from civilians, and military objectives from civilian objects. The question arises whether by making oneself invisible, rather than disguising oneself as a civilian, a soldier runs afoul of this distinction obligation.

Consider, for example, a situation where invisible soldiers launch attacks from within a civilian crowd. Without being unable to see where the attacks originate from, the adverse party may find it impossible to identify or distinguish combatants from civilians in their counter-attack and may thus be prevented from complying with the principle of distinction. Such a situation could be considered analogous to human shielding in that invisible soldiers take advantage of the presence of the civilian population to render their area of operation immune from the adversary’s attacks or impede their military operations, conduct specifically prohibited under Article 51(7) of Additional Protocol I and, as Michael Schmitt has pointed out, under customary international law in both international and non-international armed conflict.

However, the mere presence of civilians in the vicinity of covert military operations is in itself not sufficient to qualify as human shielding. As Professor Schmitt discussed previously, intent is the critical element in the prohibition of human shielding. Invisible soldiers cannot be seen as taking the civilian population as a human shield when they are simply conducting an urban warfare in a civilian populated area. Operating invisibly in the vicinity of civilians only violates the law of armed conflict if it is a purposeful action intended to put protected persons at risk of harm in order to shield the soldiers from attack or impede the adversary’s military operations (§ 5.16.2).

The prohibition of human shielding does not extend to the use of civilian objects, such as buildings and schools, to shield from attack. Thus, invisible soldiers using a civilian building as a firing position and depriving the adverse party of any chance to detect them do not violate this prohibition. Invisible soldiers may find it necessary to operate from within a civilian building to avoid detection from the excess heat released from their body after moving quicky over the terrain. In such a case, hiding behind the building wall is motivated by a desire to complicate the adversary’s task of detecting invisible soldiers, rather than exploiting the protected status of the civilian object as a means of shielding themselves from attack.

Passive Precautions

Each party to an armed conflict has the duty to take feasible precautions to separate the party’s own civilian populations and objects from the vicinity of military objectives, an obligation generally known as “passive precautions.” When using invisibility technology, belligerent parties are not free from this obligation and must endeavor to segregate cloaked military assets from civilian objects or protect them from the effects of attack.

It must be cautioned that the duty to exercise passive precautions does not require complete separation of military objectives from civilians or civilian objects all the time. The extent of this obligation is qualified to the extent feasible; in other words, taking measures that are “practicable or practically possible, taking into account all circumstances ruling at the time, including humanitarian and military considerations” (§ It is thus far-fetched to argue that any deliberate intermingling of combatants amongst civilians or civilian objects violates the obligation to take passive precautions. As is the case with human shields, the outcome of this legal assessment depends on the intention behind the practice of intermingling.

Invisible military aircraft could operate from a civilian airfield because of the strategic significance of its location or tag along after a civilian aircraft to avoid detection from the engine noise and exhaust heat. In such cases, the use of civilian infrastructure and air passage routes becomes imperative to meet the operational necessity of complicating the enemy’s task to detect and identify invisible military aircraft. As John Goehring correctly observed, intermingling with civilians and civilian objects to complicate the task of an adversary alone does not breach the duty to exercise passive precautions.

Concluding Thoughts

The prospect of gaining an ability to cloak oneself completely across the visible light spectrum edges close to reality as science and technology advance. The use of invisibility technology is expected to significantly enhance armed forces’ ability to protect themselves from an adversary’s attacks and to conduct covert military operations effectively. Cloaking does not necessarily violate existing law unless the intention behind it is to exploit the legal protection afforded under the law of armed conflict. In principle, it will be considered as a technologically enhanced form of optical deception and justified as a ruse of war.

Whether the use of invisibility technology ought to be prohibited or restricted is a different question. The idea of fighting while being invisible might cause moral revulsion for some people who are strongly motivated to uphold the chivalric code in warfare. Many Harry Potter fans must have found it exciting when he fought back against Lord Voldemort’s followers by casting curses and protective spells from underneath his invisibility cloak, whereas the “United Federation of Planets” in the world of Star Trek made a conscious decision not to develop or use cloaking technology. The latter perhaps reflects Gene Roddenberry’s philosophy of how future space warfare should be fought. Only the future will tell if many more follow his philosophy and demand forbearing the use of invisibility technology as a method of warfare, and whether they can prevail over the political quest for military superiority.


Hitoshi Nasu is a Professor of Law in the Department of Law at the United States Military Academy.




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