Symposium on Military AI and the Law of Armed Conflict: Drone Swarms as Weapons of Mass Destruction

[Jimena Sofía Viveros Álvarez serves as the Chief of Staff and Head Legal Advisor to Justice Loretta Ortiz at the Mexican Supreme Court as well as a member of the UN Secretary General’s High-Level Advisory Body on AI. Her prior roles include national leadership positions at the Federal Judicial Council, the Ministry of Security, and the Ministry of Finance, where she held the position of Director General. ]

Technological progress may aim to benefit humanity. However, it can also bring forth the emergence of new means and methods of warfare. Presently, the rapid development of artificial intelligence (AI) has ushered in a new paradigm of warfare, in which machines can make life-or-death decisions without any human agency.

Military AI use-cases range from intelligence gathering, data processing, research and development, cybersecurity, to enhanced armaments, such as autonomous weapons systems (AWS). The latter category refers to any weapon capable of executing its critical functions (that is, targeting, engaging and using force) without human intervention and which can operate in any domain, whether it is air, sea, land or space.

Depending on the sophistication of its AI, AWS can act with varying degrees of autonomy depending on the level of human-machine interaction they require to pursue their objectives:

  • Human in-the-loop: The system requires human input on the target selection and course of action.
  • Human on-the-loop: There is a human supervising and approving the system’s actions. 
  • Human off-the-loop: The system selects and engages target without any human intervention, which is also referred to as full autonomy.

Since 2013, discussions regarding the risks and possible regulation of AWS have unfolded within the framework of Convention on Certain Conventional Weapons. Nevertheless, due to lack of consensus among State Parties, the most relevant breakthrough are the 2019’s principles on AWS. Inter alia, these recognize the need to retain human responsibility, and that human-machine interaction should ensure these technologies comply with IHL.

One central element of the discussions is the level of autonomy that any weapon system should be allowed to have. Absence of human control over the system’s decision-making processes to select, engage and use lethal force, challenges the allocation of responsibility since a machine cannot be held liable. This is what is known as the accountability gap.

Moreover, it is still uncertain whether AWS can adequately adhere to the core principles of International Humanitarian Law (IHL). This is due to AI’s brittleness and/or algorithmic deficiencies, such as biases or misalignments, making the system inherently unpredictable.

Clearly, AWS pose several challenges that can only be exacerbated when they operate in large numbers. Such is the case of drone swarms, which could potentially have the impact of weapons of mass destruction (WMD) as will be discussed below.

What are Drone Swarms? 

Swarming” is a longstanding military tactic whereby several units converge and attack a specific target in a structured and coordinated manner. Hence, drone swarms refer to large groups of AWS that can share information amongst each other, synchronize, and carry out their shared objectives with high-precision, coordination, and efficiency.

What characterizes a drone swarm is not the simultaneous deployment of numerous drones per se, but rather their capability to coordinate amongst each other and act as one. To achieve this, they can use various methods of command and control that can be centralized or decentralized. The first occurs when a human operator controls each drone from a ground station, whereas the latter grants autonomy to the swarm to collaborate, adapt and act based on the information they share.

A swarm is not necessarily composed of identical drones. Rather, it can include different units with distinct characteristics and functionalities. In such cases, separate roles may be assigned to the various components of the swarm, for example, some may be suited for surveillance and intelligence, while others may be offensive in nature.

One of the earliest showcases of these capabilities dates back to 2016, when the United States (US) tested 103 Perdix drones at a demonstration in California. However, it was not until May 2021 that Israel became the first country to openly deploy a drone swarm that gathered intelligence on the geographical position of Hamas militants and relayed this information to ground-based missiles to strike targets located miles away. 

There is a notable interest in the development of drone swarms, with several countries such as China, France, India, Israel, Russia, South Korea and the US creating programs for their development, which risks their proliferation at a larger scale. 

The author argues that given their scalability of up to thousands of units, which exponentially increases their capability for harm and mass casualty rates, drone swarms virtually amount to WMD.

Drone Swarms as Weapons of Mass Destruction

The United Nations (UN) General Assembly’s Resolution A/RES/32/84-B of 1977, considers WMDs as: “[…] atomic explosive weapons, radioactive material weapons, lethal chemical and biological weapons, and any weapons developed in the future which might have characteristics comparable in destructive effect to those of the atomic bomb or other weapons mentioned above.” 

While this enunciative definition recognizes that WMD are not limited to chemical, biological, radioactive, and nuclear (CBRN), it in fact does not clarify what exactly separates them from other conventional weapons. Thus, to have some precision on their characteristics and understand whether drone swarms can classify as such, it is relevant to analyze the International Court of Justice’s (ICJ) case-law.

In the Advisory Opinion concerning the Legality of the Threat or Use of Nuclear Weapons of 1996, Judge Shahabuddeen stated in his Dissenting Opinion that these weapons are unacceptable due to their destructive capability to “kill millions” with a single use, as well as their inability to abide by IHL standards.

Additionally, in the Case on the Obligations Concerning Negotiations Relating to Cessation of the Nuclear Arms Race and to Nuclear Disarmament of 2016, the Dissenting Opinion of the late Judge Cançado Trindade, notes that WMD cause unnecessary suffering and their effects cannot be limited to military targets, that is, that they are indiscriminate and disproportionate.

It is an established principle of customary international law that weapons which are incapable of distinguishing between civilian and military targets are prohibited and that their use would amount to an indiscriminate attack, banned under customary law and under article 51(4)(a) of Additional Protocol I (API) of 1977 to the Geneva Conventions of 1949.

On the other hand, the principle of proportionality (article 51(5)(b)) refers to the proscription to cause excessive incidental or involuntary harm to the civilian population in relation to the military advantage anticipated. Notably, a direct attack on civilians, in violation of the principle of distinction (API article 48, 51(2), 52(2)), may be inferred from the indiscriminate character of the weapon used, particularly if the “incidental” harm is excessive. 

The scalability of drone swarms, that is, their possibility to vary in size, ranging from just a few units to thousands, is what increases their potential for harm. Thus, a drone swarm becomes a WMD when it is large enough to meet the same threshold as other CBRN armaments in terms of destructive capabilities, regardless of the munition used. 

On this note, the author argues that large drone swarms will be inherently indiscriminate and disproportionate. Once the swarms scale in the thousands, its operator’s cognitive capacity would simply fail to meaningfully control the sheer number of drones. This loss of control makes the drones intrinsically unpredictable, causing failures to be infinitely possible and, in some instances, even inevitable.

The author proposes that any WMD would also satisfy the contextual elements for crimes against humanity which can be committed both, during peacetime or armed conflict, as opposed to war crimes which can only be committed during an international or non-international armed conflict. In any case, what is required is that the attack is directed against the civilian population and meets the criterion of widespread or systematic as defined by the Elements of the Crimes of the International Criminal Court and its jurisprudence, as well as that of the ad hoc tribunals. The term “systematic” refers to the organized nature of the acts and the unlikelihood of their random occurrence, whereas “widespread” refers to the large-scale nature of the attack. 

This is relevant as drone swarms could be easily (mis)used to carry out large scale attacks against the civilian population, and not necessarily in the context of an armed conflict for IHL to apply. Considering their scalability, drone swarms could cause immeasurable damage akin to other WMD. Thus, their deployment would amount to an international crime, de facto and de jure, since by definition, WMD cause widespread harm.

Although currently there is no specific threshold on the required level of destruction for any weapon to be considered a WMD, the scalability of drone swarms could easily meet any of the generally accepted criteria in this regard. Granted, a small cluster of drones may not be considered as WMD, unless they are equipped with CBRN. Actually, in said scenario those drones would only be vehicles, not WMD themselves.

Developers or proponents of these technologies may argue that the AI controlling the swarm can indeed distinguish between military and civilian objectives. However, the author purports that AI’s inherent biases, misalignments and brittleness, render them unpredictable and risk the occurrence of indiscriminate attacks, whether intentional or not, and thus would never pass a weapons review under article 36 of (API). In fact, given the continuous communications between each drone, a single unit malfunctioning could be catastrophic, as it could relay erroneous information to the swarm, for example, by mistakenly selecting a school bus as a target, and prompt all other drones to follow its lead and attack multiple school buses.

As will be discussed below, classifying drone swarms as WMD would be an major milestone to prevent their harmful consequences.

Importance of Categorizing Drone Swarms as WMD

Even if there is no immediate restriction arising from the recognition of drone swarms as a WMD, this concept bears a stigma that may deter States from deploying them to avoid condemnation. 

Additionally, as noted by the ICJ in its Nuclear Weapons Advisory Opinion, since the origins of the UN, disarmament efforts, especially vis-á-vis WMD, have played a pivotal role to ensure international peace and security. Notoriously, these have led to the adoption of legally binding instruments prohibiting the use of WMD, such as the Biological Weapons Convention (1972); the Chemical Weapons Convention (1992); the Treaty on the Non-Proliferation of Nuclear Weapons (1968); and most recently the Treaty in the Prohibition of Nuclear Weapons (2017). Moreover, the UN Security Council’s Resolution 1540 (2004) calls upon States to prevent the proliferation of WMD and collaborate with the relevant international organizations to achieve this objective.

Furthermore, existing Treaties such as the Outer Space Treaty or the Seabed Treaty that prohibit the placing of nuclear or “other weapons of mass destruction” in the seabed and space, respectively, would immediately be applicable to drone swarms if they are classified as a WMD.

Moreover, while the Rome Statute does not criminalize WMD, early negotiations saw proposals to include the use of these weapons as a war crime, but they unfortunately were not included in the final stages, although article 8(2)(b)(xx) does mention a prohibition of weapons and methods of warfare which are inherently indiscriminate. However, this is subject to a comprehensive list that should be included as an annex by an amendment, which regrettably has not been agreed upon yet.

Therefore, acknowledging drone swarms as WMD would serve as deterrence against their development and deployment, aiming at eventually raising consensus for a binding instrument regulating or even banning them.


Drone swarms are the newest weapons of mass destruction. Due to their scalability, they have the potential to meet any threshold in terms of mass destruction. Furthermore, the impossibility of a human operator to effectively control every action of the swarm renders them prone to be indiscriminate and disproportionate. 

Thusly, their deployment during any armed conflict, whether international or not, would likely result in the commission of war crimes, and during peacetime their use would meet the threshold of a widespread attack, configurating the commission of crimes against humanity. 

Conclusively, the implications of categorizing drone swarms as WMD are not minor. This classification would deter or at least caution States to approach the use of this attack method with the necessary precautions, and highlight the importance of their regulation, or ideally prohibition. In this regard, last October, the UN Secretary General and the International Committee of the Red Cross issued a landmark joint call urging States to launch negotiations of a new legally binding instrument to set clear prohibitions and restrictions on AWS by 2026. The author largely celebrates this visionary effort to protect all of humanity from these technologies, and further proposes that said negotiations aim to include a ban on drone swarms, to prevent the proliferation of yet another WMD.

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Jimena Viveros