A State’s Legal Duty to Warn its own Civilians on Consequences of Direct Participation in Hostilities

by | Feb 21, 2023

App duty to warn

Much has been written – both in the media and on this site – about the innovative and tactically-effective use of a new digital app, ePPO, by Ukrainian civilians in defense against Russia’s unlawful war of aggression. This app is accessible by personal cell phone and allows civilian users to quickly identify, locate, and track the trajectory of airborne Russian military threats like helicopters, jet aircraft, rockets, and drones, and send those data to Ukrainian armed forces, including air defense batteries. Those forces receive these data in time to respond to the threat the military might not have otherwise detected in time.

The app’s most obvious legal issue is when its use constitutes a civilian’s “direct participation in hostilities.” As Michael Schmitt and William Biggerstaff wrote last Fall (here and here) about this implication, direct participation temporarily suspends the protection afforded to civilians under international law and makes them a lawful target of the Russian armed forces.

Their analysis is correct on the law, so this post asks a novel follow-up question. Does the government of Ukraine, under these conditions, owe the user of this app a warning, a legally required caveat emptor to put the civilians on notice that even this ostensibly passive action leaves her open to not just to any Russian attack, but a lawful one?

The answer, explained below, is no – at least not explicitly under international law. But a reasonable interpretation of current rules suggests such a duty can be implied and easily satisfied. A State acting in good faith, consistent with the scope of relevant treaty provisions and consistent with obligations owed under the law of State Responsibility, will accept and conform to that duty.

This question is relevant beyond the specifics of the ePPO app. Therefore, this post will initially broaden the point of view and suggest a hypothetical. After the hypothetical prompt, and a review of the relevant international law, we will return to the Ukrainian app scenario.

Hypothetically Speaking

Eight months after Country A invaded and attacked military and civilian targets in Country B, the war has come to a bitter and bloody stalemate. No side seems to have a distinct advantage: the invading Country A has more advanced and better equipped military hardware and more troops than Country B; Country B fights an existential battle for its right to exist as a sovereign power and relies on a loose confederation of friendly nations to supply it with arms and equipment, the high morale of its citizens, the aggressive creativity of its smaller armed forces, and the charismatic and optimistic leadership of its elected government. For the most part, Country A’s attacks on civilian locations and objects are launched “over-the-horizon,” relying on armed drones and rockets rather than pushing poorly led and unmotivated ground forces into urban areas now strewn with man-made obstacles and fierce resistance from the population.

After mobilizing its reserves, and declaring martial law, the government of Country B opted not to enact a nationwide draft. Nevertheless, Country B’s ministry of defense has regularly encouraged its largest defense contractor, a small arms manufacturer, to freely distribute rifles and ammunition to civilians who have chosen to remain in areas close to the active hostilities. And the government has supplemented this “arms drive” with a new contract term that financially incentivizes the company for doing so. Through online social media, television, and radio, government officials routinely encourage their fellow citizens in these areas to “support the cause,” “defend your homes and families,” and “fight for your freedom” whenever and wherever they come across enemy armed forces. Both Country A and Country B have ratified the Geneva Conventions and the Additional Protocols of 1977.

The Problem and the Law

This hypothetical can be boiled down to the purposeful arming of civilians with the means and motivation to engage in unplanned attacks against the military adversary at opportunistic times and places of the civilian’s choosing. This post asks whether – or under what specific conditions – a nation’s intentional supply to civilians of the means to frustrate, impede, or damage enemy military objects or personnel triggers a prefatory duty to warn those civilian recipients that their use of those means likely makes them lawfully targetable combatants from the perspective of not only the enemy but also international law. Addressing the question requires us to shift perspective from what the enemy military may do with its temporary legal license to kill (which is largely uncontroversial) to what the civilian’s own government must do given the possibility of that license.

The Law of Armed Conflict or International Humanitarian Law – whether the belligerent is a member of a nation’s armed forces or is an “unlawful enemy combatant” or “unprivileged belligerent” – is ultimately grounded in key principles of necessity, distinction, proportionality, and humanity. Codified by treaty, the customary norm and principle of distinction embodies the humanitarian obligation to distinguish combatant from non-combatant during military operations. The principle’s aim is to ensure “protected” people and property do not become the deliberate targets of a planned military strike (“attacks” including both defensive and offensive military actions). But the duty to distinguish is a relatively simple matter only under a limited range of conditions: when Country A’s armed forces are in a pitched battle against Country B’s armed forces in its unpopulated regions far from schools, hospitals, markets, sports stadiums, residential apartment blocks, museums, commercial centers, and governmental offices and each side’s armed forces wear uniforms.

The matter becomes decidedly more difficult, and exceptionally controversial, the closer the fighting comes to civilians. Not only is there a parallel concern with mitigating unnecessary “collateral damage” under the principle of proportionality, but there is also the increased risk that civilians will take up arms in self-defense or more actively and directly participate in hostilities as if they are part of a regular militia or as members of an organized armed group.  This difficulty is addressed with an entire legal doctrine and extensive scholarly literature devoted to explaining “direct participation in hostilities.”

Such participation suspends the protection of the individual civilian from intentional targeting. This belligerent status-through-conduct question is more than academic: the conduct opens the person to the prospect of being made the lawful target of a direct attack. Under the “direct participation in hostilities” rule, those civilians actively taking up arms lose their protections under international law and become lawfully targetable, at least for the time in which they are aiming and firing those weapons, immediately preparing to do so, and in departing the area of the attack (and possibly for a much longer duration both before they take aim and after they fire off rounds, depending on how frequent, repeated, or militarily significant this conduct is).

Addressing the contested borders of the “direct participation in hostilities” rule (i.e., the temporal and geographic proximity elements of “direct causation”), the lawfulness of arming civilians, and the lawfulness of civilians taking up arms at all are beyond the scope of this post.  Rather, I will assume for the sake of this argument that what the citizens of Country B are doing – when they carry arms provided by the government and engage in sua sponte attacks on the military of Country A – is “direct participation in hostilities.” The question that needs more thought, however, is whether Country B owed its population a special warning before providing those arms and encouraging their use. The warning is not that such use is inherently dangerous, as if it were a corporation’s product liability warning to stave off civil lawsuits, but instead a warning that such use strips the proactive civilian of an important protection under international law.

Part of the overarching obligation to protect civilians and non-combatants from the threat of armed violence during conflict is an exercise of feasible precautions, as embodied in Article 58 of the first Additional Protocol to the Geneva Conventions (“precautions against the effects of attacks”). It holds that “the parties shall . . . to the maximum extent feasible . . . take other necessary precautions to protect the civilian population, individual civilians and civilian objects under their control against the dangers resulting from military operations.” The U.S. Department of Defense’s Law of War Manual (§ 5.14) recognizes that this obligation is considered a binding duty, even in the absence of a specific task enumerated in a treaty, as part of customary international law. If the text itself creates an ambiguity as to which civilian population it protects – that of the adversary’s nation or that of the country responsible for conducting the attack – then consider the ICRC’s 1987 Commentary on Article 58:

It is not concerned with laying down rules for the conduct to be observed in attacks on territory under the control of the adversary, but with measures which every Power must take in its own territory in favour of its nationals, or in territory under its control. (emphasis added)


Belligerents may expect their adversaries to conduct themselves fully in accordance with their treaty obligations and to respect the civilian population, but they themselves must also cooperate by taking all possible precautions for the benefit of their own population as is in any case in their own interest. (emphasis added)

This duty is imposed both on nations using force against others and those defending their territory and people from such attacks. Whether we view civilians of Country B using government-provided rifles and bullets as a purely defensive act or as an offensive act, the result is the same.

Moreover, Article 57(1) (“precautions in attack”) of Additional Protocol I states: “In the conduct of military operations, constant care shall be taken to spare the civilian population, civilians and civilian objects.” Article 57(2)(c) demands even more than constant care: “effective advance warning shall be given of attacks which may affect the civilian population, unless circumstances do not permit.” While a conventional reading of this requirement suggests a warning to the local population in an area about to be attacked, a broader reading that encompasses civilians who are participating in hostilities cannot be immediately dismissed: after all, even a civilian who loses protection through direct participation in hostilities still remains a “civilian.”

Nevertheless, such a broad interpretation of Article 57(2) may be beyond the drafters’ intentions. For example, Article 57(2)(a)(ii) and (iii) requires States to “take all feasible precautions in the choice of means and methods of attack with a view to avoiding, and in any event to minimizing, incidental loss to civilian life . . . ” and to “refrain from deciding to launch any attack which may be expected to cause incidental loss of civilian life.” (Emphasis added).  An initial reading suggests that it mirrors the obligation in Article 58 (“to the maximum extent feasible . . . take other necessary precautions”). However, we should be attentive to the double use of the word “incidental” to describe the loss we want to prevent or limit. Once a person engages in the “direct participation of hostilities,” a logical consequence of her loss of protection is that any lawful strike aimed at her need not consider her in the pre-attack estimation of possible collateral damage or harm. Loss of protected status by definition means that she is no longer potential collateral damage at all, but rather the direct object of an otherwise legal attack.

Because the text of Article 57(2) narrowly shields only those civilians who might be considered collateral damage, any proponent of a State duty to warn civilians who directly participate in hostilities must fall back only on the more generalizable and generic applicability of Article 57(1) and Article 58 (neither one of which seem to care about what the civilians may be doing, as long as there are military operations ongoing). A universal duty to protect along these lines is supported by the ICRC’s Commentary on this rule as well: “Article 57 applies to all attacks, whether they are acts of aggression or a response to aggression.”

Finally, it should be noted that the law of war also demands that nations must train their combatants on their obligations and constraints under international law, necessarily including the conditions under which they lose their lawful combatant status. The ICRC’s Customary International Law Study, Rule 142 says this “norm” is applicable to the “armed forces” – a term that “must be understood in its generic meaning.” Looking more specifically to the Geneva Conventions (e.g., GC I, art. 47 and Additional Protocol I, art. 83), nations must not only “disseminate the text” of the Conventions, but also  “include the study thereof in their programmes of military and, if possible, civil instruction, so that the principles thereof may become widely known to the entire population . . .” (emphasis added).

Ukraine’s ePPO and the Duty to Warn

The Country A vs. Country B hypothetical may seem implausible, structured with so many constraining facts and a crowdsourcing of national defense that it looks like a law school exam prompt. But the real-world Ukrainian ePPO app and its use by civilians is nearly the same. It is an app that has no other functionality or purpose than to identify, track, and transmit data about enemy weapons of war in real time (even a rifle can be used to hunt for food or self-defense; this app serves no secondary purpose). It is an app that may be freely downloaded onto a private cell phone; it is free both in cost and not demanded by the Ukrainian government edict. As long as the app is first accessed through another established Ukrainian government app or online portal (that requires users to be eighteen years or older to access, and development of which the U.S. government helped fund), any Ukrainian adult may voluntarily use this app at their discretion if and when they believe they have spotted a hostile Russian airborne threat.

The information  – by its nature – is time sensitive. It is instantly transmitted to currently active air defense units nearby that have the capacity to defeat the threat in a way that the civilian cannot. That information is then used to aim an anti-aircraft cannon, rocket, or missile which is then fired at the incoming threat. Even if the air defense unit relied primarily on other sources for its tracking data, and simply used the ePPO signals to confirm its own, the civilian’s partial contribution to a direct attack is probably enough to make her a direct participant in hostilities. In such a case, the civilian’s pointing of her cell phone toward the observed object, clicking the button, and sending the information satisfies even the ICRC’s description of direct participation:“transmitting tactical targeting intelligence for a specific attack” is an example of “causing military harm to another party.” As Schmitt and Biggerstaff noted, it is just a modern day version of smoke signaling and rooftop spotting, acts universally believed to be hostile when connected to a near imminent attack on an enemy target relying at least in part on that information.

The Ukrainian Ministry of Defense actively promoted the app, helped (at least to some extent) design it, and has relied on subsequent civilian use to launch strikes on incoming aircraft and drones. Use of the app, as it is intended to be used, unequivocally strips the civilian user of her protection from targeting at least for some period (depending on how the duration of “participation” is interpreted). Ukraine did everything, it seems, to operationalize this novel tool of modern warfare except code the app itself and upload it to the personal devices of an unsuspecting population. Ukraine did not do one other thing, however. It did not warn the population of the legal consequences of their direct participation in hostilities.

Consider what would be required if a civilian nurse, inside a fixed hospital treating both military and civilian wounded and sick, had downloaded the app and used it one afternoon when he saw an Iranian Shahed-136 drone fly overhead. At least for the time during which the nurse is opening the app, aiming the cell phone at the drone, and transmitting the data to a Ukrainian air defense battery, he is directly participating in hostilities. If Russia had the means to identify this app in use by the nurse in the hospital, Russia could – lawfully – strike the nurse to prevent this or foreseeably future harmful acts on Russian military forces. But because the nurse is inside a protected place (a hospital – see Article 18 of GC IV), Russia must first warn (Art. 19, GC IV) the Ukrainians that a strike is forthcoming if the direct participation in hostilities continues or if the hospital continues to be used as a location from which to spot aircraft and aid the military defenders. That duty to warn is uncontroversial. If Russia would be so obligated, why would Ukraine not be when it actively promotes and provides the reason that triggered Russia’s own obligation?

A Feasible Precaution Under the Circumstances

To restate, there is no explicit affirmative duty to warn one’s own civilians in this way under the express terms of international humanitarian law (whether such a duty could be inferred by International Human Rights Law, the Art. 2 “right to life” under the Convention for the Protection of Human Rights and Fundamental Freedoms, is beyond the scope of this post).  A fair reading, however, of those terms suggests that such a duty should be imposed at least under certain conditions, especially in light of the rapid advance of technology and everyday access to it. Specifically, a proponent for such a duty should concentrate on two sources:

– the obligations to exercise feasible precautions under Articles 57 and 58 of the Additional Protocol I;

– the general obligation to respect and ensure respect for the law of armed conflict under Common Article 1 of the Geneva Conventions and under customary international law, requiring the State to educate its entire population – both military agents and civilians – on the law of war.

The conditions under which this duty ought to arise are straightforward. If a State intentionally provides its civilians with the means, or encouragement to use means, of disabling, frustrating, impeding, or destroying or otherwise harming military capabilities or military objectives of another State, the State shall take all feasible measures to clearly inform its civilians that such use may trigger their loss of protection under international law and thus makes them subject to lawful attack. State action that prompts, promotes, encourages, or incentivizes civilian conduct that equates to “direct participation in hostilities,” or State action or inaction such that civilian conduct may be attributed to the State itself under the law of State Responsibility, triggers this affirmative duty to warn. If this standard is applied to Ukraine’s endorsement of the ePPO app, then Ukraine owes its citizen air defenders a simple warning.

Articulating that warning would be an easy thing to do in the app itself – a simple, blunt legal disclaimer the user must read before downloading the app. Something like: “WARNING: USE OF THIS APP FOR ITS INTENDED PURPOSES MAY CONSTITUTE ‘DIRECT PARTICIPATION IN HOSTILITIES’ AND TRIGGER YOUR LOSS OF PROTECTION FROM ARMED ATTACK UNDER INTERNATIONAL LAW.”  The warning may or may not chill the citizen’s initial enthusiasm to take an active role in defending her town or neighborhood. In a crisis of the scale facing the citizens of Ukraine, it is doubtful that a warning like this would stop anyone. But even if a significant number of citizens would be frightened off by the warning and choose not to download the app, that risk is not a relevant factor in determining and implementing a State’s legal obligation. In fact, it may be even more of an important moral demand for the State to ensure that those citizens who would choose not to use the app for this reason are given the opportunity to make that informed choice.


Dan Maurer, Lieutenant Colonel, Judge Advocate, U.S. Army, serves as professor of national security law at The Judge Advocate General’s Legal Center and School.  This post reflects the author’s own opinions and not the official positions of the U.S. government.  The author wishes to thank Major Olesea Roan, LL.M. candidate at The Judge Advocate General’s Legal Center and School, for translation assistance with the ePPO app., and wishes to thank Sean Watts, Michael Schmitt, Hitoshi Nasu, Grant McDowell, and Jennifer Maddocks for insightful comments on earlier drafts, and appreciates his colleagues in the school’s National Security Law department and the judge advocate students in his war crimes class with whom he debated and discussed this possible duty-to-warn while writing this article.


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