Robotics has extensively contributed to modify defense systems. Significant examples from the recent past include teleoperated robots detecting and defusing explosive devices (e.g., PackBot) [1], in addition to unmanned vehicles for reconnaissance and combat missions, operating on the ground (e.g., Guardium [2] or TALON [3]) or in the air (e.g., MQ-1 Predator [4]). The deployment of these military robots has been seldom objected to on ethical or legal grounds, with the notable exception of extraterritorial targeted killings accomplished by means of unmanned aerial vehicles. These targeted killings have raised concerns about the infringement of other States’ sovereignty and overly permissive application of lethal force in counter-terrorism operations [5,6,7].

One should carefully note that the release of destructive force by any weaponized robot in the above list is firmly in the hands of human operators. Accordingly, ethical and legal controversies about these systems were confined to a handful of their specific uses, and their overall acceptability as weapons systems was never questioned. However, the entrance on the scene of autonomous weapons systems (AWS from now on) has profoundly altered this ethical and legal landscape.

To count as autonomous, a weapons system must be able to select and engage targets without any human intervention after its activation [8••, 9, 10]. Starting from this basic and quite inclusive condition, the Stockholm International Peace Research Institute (SIPRI) [11] introduced additional distinctions between types of existing AWS: (i) air defense systems (e.g., Phalanx [12], MANTIS [13], Iron Dome [14], Goalkeeper [15]); (ii) active protection systems, which shield armored vehicles by identifying and intercepting anti-tank missiles and rockets (e.g., LEDS-150 [16] and Trophy [17]); (iii) robotic sentries, like the Super aEgis II stationary robotic platform tasked with the surveillance of the demilitarized zone between North and South Korea [18]; (iv) guided munitions, which autonomously identify and engage targets that are not in sight of the attacking aircraft (e. g., the Dual-Mode Brimstone [19]); and (v) loitering munitions, such as the Harpy NG [20], which overfly an assigned area in search of targets to dive-bomb and destroy.

This classification stands in need of continual expansion on account of ongoing military research projects on unmanned ground, aerial, and marine vehicles that are capable of autonomously performing targeting decisions. Notably, research work based on swarm intelligence technologies is paving the way to swarms of small-size and low-cost unmanned weapons systems. These are expected to overwhelm enemy defenses by their numbers and may additionally perform autonomously targeting functions [21,22,23,24].

The technological realities and prospects of AWS raise a major ethical and legal issue: Is it permissible to let a robotic system unleash destructive force and take attendant life-or-death decisions without any human intervention? This issue prompted intense and ongoing debates, at both academic and diplomatic levels, on the legality of AWS under international law [25]. An idea that has rapidly gained ground across the opinion spectrum in this debate is that all weapons systems, including autonomous ones, should remain under meaningful human control (MHC) in order to be ethically acceptable and lawfully employed (see the reports by the UK-based NGO Article 36 [26, 27]). Nevertheless, the precise normative content of such requirement is still far from being precisely spelled out and agreed upon.

This review provides a general survey of the AWS debate, focusing on the MHC turning point and its ethical and legal underpinnings. After recalling the initial stages of the debate, a schematic account is provided of chief ethical and legal concerns about autonomy in weapons systems. Then, the main proposals regarding the MHC content are introduced and analyzed, including our own proposal of a “differentiated and prudential” human control policy on AWS. Finally, it is pointed out how our proposal may help overcome the hurdles that are currently preventing the international community from adopting a legal regulation on the matter.

Highlights from the AWS Ethical and Legal Debate

Members of the robotics community, notably Ronald C. Arkin and Noel Sharkey, were chief protagonists of early discussions about the ethical and legal acceptability of AWS. Arkin emphasized some ethical pros of autonomy in weapons systems. He was concerned about the poor record of human compliance with international norms governing the conduct of belligerent parties in warfare (Laws of War or international humanitarian law (IHL)). In his view, this state of affairs ultimately depends on human self-preservation needs and emotional reactions in the battlefield—fear, anger, frustration, and so on—that a robot is immune to. Arkin’s own research on military applications of robotics was inspired by a vision of “ethically restrained” autonomous weapons systems that are capable of abiding “by the internationally agreed upon Laws of War” better than human warfighters. He presented this vision and its ethical motivations in an invited talk at the First International Symposium on Roboethics, organized by Scuola di Robotica, chaired by Gianmarco Veruggio, and held in 2004 at Villa Alfred Nobel in Sanremo, Italy. Arkin later described this meeting as “a watershed event in robot ethics” [28••, 29, 30].

In contrast with Arkin’s views, Sharkey emphasized various ethical cons of autonomy in weapons systems. He argued that foreseeable technological developments of robotics and artificial intelligence (AI) offer no support for the idea of autonomous robots ensuring a better-than-human application of the IHL principles. He emphasized that interactions among AWS in unstructured warfare scenarios would be hardly predictable and fast enough to bring the pace of war beyond human control. And he additionally warned that AWS threaten peace at both regional and global levels by making wars easier to wage [31,32,33,34]. Sharkey co-founded the International Committee for Robot Arms Control (ICRAC) in 2009 and played a central role in creating the conditions for launching the Campaign to Stop Killer Robots. This initiative is driven by an international coalition of non-governmental organizations (NGOs), formed in 2012 with the goal of “preemptively ban[ning] lethal robot weapons that would be able to select and attack targets without any human intervention.”

A similar call against “offensive autonomous weapons beyond meaningful human control” was made in the “Open Letter from AI & Robotic Researchers,” released in 2015 by the Future of Life Institute and signed by about 4500 AI/robotics researchers and more than 26,000 other persons, including many prominent scientists and entrepreneurs. Quite remarkably, the Open Letter urges AI and robotics researchers to follow in the footsteps of those scientists working in biology and chemistry, who actively contributed to the initiatives that eventually led to international treaties prohibiting biological and chemical weapons [35].

Worldwide pressures from civil society prompted States to initiate discussion of normative frameworks to govern the design, development, deployment, and use of AWS. Diplomatic dialogs on this topic have been conducted since 2014 at the United Nations in Geneva, within the institutional framework of the Convention on Certain Conventional Weapons (CCW). The CCW’s main purpose is to restrict and possibly ban the use of weapons that are deemed to cause unnecessary or unjustifiable suffering to combatants or to affect civilians indiscriminately. Informal Meetings of Experts on lethal autonomous weapons systems were held on an annual basis at the CCW in Geneva, from 2014 to 2016. Subsequently, the CCW created a Group of Governmental Experts (GGE) on lethal autonomous weapons systems (LAWS), which still remains (as of 2020) the main institutional forum where the issue of autonomy in weapons systems is annually debated at an international level [36]. Various members of the robotics research community take part to the GGE’s meetings. So far, the main outcome of the GGE’s work is the adoption by consensus of a non-binding instrument, that is, the 11 Guiding Principles on LAWS, which include broad recommendations on human responsibility (Principles (b) and (d)) and human-machine interaction (Principle (c)) [37].

A clear outline of the main ethical and legal concerns raised by AWS is found already in a 2013 report, significantly devoted to “lethal autonomous robotics and the protection of life,” by the UN Special Rapporteur on extrajudicial, summary, or arbitrary executions, Christof Heyns [38••]. These concerns are profitably grouped under four headings: (i) compliance with IHL, (ii) responsibility ascription problems, (iii) violations of human dignity, and (iv) increased risk for peace and international stability. Let us briefly expand on each one of them, by reference to relevant sections in Heyns’ report.

  1. (i)

    Compliance with IHL would require capabilities that are presently possessed by humans only and that no robot is likely to possess in the near future, i.e., to achieve situational awareness in unstructured warfare scenarios and to formulate appropriate judgments there (paras. 63–74) (in the literature, see [39,39,41] for a critique of this argument and [42,42,44] for a convincing rejoinder).

  2. (ii)

    Autonomy in weapons systems would hinder responsibility ascriptions in case of wrongdoings, by removing human operators from the decision-making process (paras. 75–81) (for further discussion, see [45,45,47]).

  3. (iii)

    The deployment of lethal AWS would be an affront to human dignity, which dictates that decisions entailing human life deprivation should be reserved to humans (paras. 89–97) (see [48,48,50] for more in-depth analysis, as well as [51] for a critical perspective).

  4. (iv)

    Autonomy in weapons systems would threaten in special ways international peace and stability, by making wars easier to wage on account of reduced numbers of involved soldiers, by laying the conditions for unpredictable interactions between AWS and their harmful outcomes, and by accelerating the pace of war beyond human reactive abilities (paras. 57–62) (this point has been further elaborated in [52]).

These sources of concern jointly make the case for claiming that a meaningful human control (MHC) over weapons systems should be retained exactly in the way of their critical target selection and engagement functions. Accordingly, the notion of MHC enters the debate on AWS as an ethically and legally motivated constraint on the use of any weapons systems, including autonomous ones. The issue of human-robot shared control in warfare is thereby addressed from a distinctive humanitarian perspective, insofar as autonomous targeting may impinge, and deeply so, upon the interests of persons and groups of persons that are worthy of protection from ethical or legal standpoints.

But what does MHC more precisely entail? What is normatively demanded to make human control over weapons systems truly “meaningful”? The current debate about AWS, which we now turn to consider, is chiefly aimed to provide an answer to these questions.

Uniform Policies for Meaningful Human Control

The foregoing ethical and legal reasons go a long way towards shaping the content of MHC, by pinpointing general functions that should be prescriptively assigned to humans in shared control regimes and by providing general criteria to distinguish perfunctory from truly meaningful human control. More specifically, the ethical and legal reasons for MHC suggest a threefold role for human control on weapons systems to be “meaningful.” First, the obligation to comply with IHL entails that human control must play the role of a fail-safe actor, contributing to prevent a malfunctioning of the weapon from resulting in a direct attack against the civilian population or in excessive collateral damages [53••]. Second, in order to avoid accountability gaps, human control is required to function as accountability attractor, i.e., to secure the legal conditions for responsibility ascription in case a weapon follows a course of action that is in breach of international law. Third and finally, from the principle of human dignity respect, it follows that human control should operate as a moral agency enactor, by ensuring that decisions affecting the life, physical integrity, and property of people (including combatants) involved in armed conflicts are not taken by non-moral artificial agents [54].

But how are human-weapon partnerships to be more precisely shaped on the basis of these broad constraints? Several attempts to answer this question have been made by parties involved in the AWS ethical and legal debate. The answers that we turn to examine now outline uniform human control policies, whereby one size of human control is claimed to fit all AWS and each one of their possible uses. These are the “boxed autonomy,” “denied autonomy,” and “supervised autonomy” control policies.

The boxed autonomy policy assigns to humans the role of constraining the autonomy of a weapons system within an operational box, constituted by “predefined [target] parameters, a fixed time period and geographical borders” [55]. Accordingly, the weapons system would be enabled to autonomously perform the critical functions of selecting and engaging targets, but only within the boundaries set forth by the human operator or the commander at the planning and activation stages [56,56,58].

The boxed autonomy policy seems to befit a variety of deliberate targeting situations, which involve military objectives that human operators know in advance and can map with high confidence within a defined operational theater. It seems, however, unsuitable to govern a variety of dynamic targeting situations. These require one to make changes on the fly to planned objectives and to pursue targets of opportunity. The latter are unknown to exist in advance (unanticipated targets) or else are not localizable in advance with sufficient precision in the operational area (unplanned targets). Under these conditions, boxed autonomy appears to be problematic from a normative perspective, insofar as issues of distinction and proportionality that one cannot foresee at the activation stage may arise during mission execution.

By the same token, a boxed autonomy policy may not even suffice to govern deliberate targeting of military objectives placed in unstructured warfare scenarios. To illustrate, consider the loitering munition Harpy NG, endowed with the capability of patrolling for several hours a predefined box in search of enemy targets satisfying given parameters. The conditions licensing the activation of this loitering munition may become superseded if civilians enter the boxed area, erratic changes occur, or surprise-seeking intentional behaviors are enacted [59]. Under these various circumstances, there is “fail-safe” work for human control to do at the mission execution stage too.

In sharp contrast with the boxed autonomy policy, the denied autonomy policy rules out any autonomy whatsoever for weapons systems in the critical targeting function and therefore embodies a most restrictive interpretation of MHC [60]. Denied autonomy undoubtedly fulfills the threefold normative role for human control as fail-safe actor, accountability attractor, and moral agency enactor. However, this policy has been sensibly criticized for setting too high a threshold for machine autonomy, in ways that are divorced from “the reality of warfare and the weapons that have long been considered acceptable in conducting it” [61]. To illustrate this criticism, consider air defensive systems, which autonomously detect, track, and target incoming projectiles. These systems have been aptly classified as SARMO weapons, where SARMO stands for “Sense and React to Military Objects.” SARMO systems are hardly problematic from ethical and legal perspectives, in that “they are programmed to automatically perform a small set of defined actions repeatedly. They are used in highly structured and predictable environments that are relatively uncluttered with a very low risk of civilian harm. They are fixed base, even on Naval vessels, and have constant vigilant human evaluation and monitoring for rapid shutdown” [62].

SARMO systems expose the overly restrictive character of a denied autonomy policy. Thus, one wonders whether milder forms of human control might be equally able to strip the autonomy of weapons systems of its ethically and legally troubling implications. This is indeed the aim of the supervised autonomy policy, which occupies a middle ground between boxed and denied autonomy, insofar as it requires humans to be on the loop of AWS missions.

As defined in the US DoD Directive 3000.09 on “Autonomy in Weapons Systems,” human-supervised AWS are designed “to provide human operators with the ability to intervene and terminate engagements, including in the event of a weapon system failure, before unacceptable levels of damage occur” (p. 13). Notably, human-supervised AWS may be used for defending manned installations and platforms from “attempted time-critical or saturation attacks,” provided that they do not select “humans as targets” (p. 3, para. 4(c)(2); see, e.g., the Phalanx Close-In Weapons System in use on the US surface combat ships). While undoubtedly effective for these and other warfare scenarios, supervised autonomy is not the silver bullet for every ethical and legal concern raised by AWS. To begin with, by keeping humans on-the-loop, one would not prevent faster and faster offensive AWS from being developed, eventually reducing the role of human operators to a perfunctory supervision of decisions taken at superhuman speed while leaving the illusion that the human control requirement is still complied with [63]. Moreover, the automation bias—the human propensity to overtrust machine decision-making processes and outcomes—is demonstrably exacerbated by a distribution of control privileges that entrusts humans solely with the power of overriding decisions autonomously taken by the machines [64].

To sum up, each one of the boxed, denied, and supervised autonomy policies provides useful hints towards a normatively adequate human-machine shared control policy for military target selection and engagement. However, the complementary defects of these uniform control policies suggest the implausibility of solving the MHC problem with one formula, to be applied to all kinds of weapons systems and to each one of their possible uses. This point was consistently made by the US delegation at GGE meetings in Geneva: “there is not a fixed, one-size-fits-all level of human judgment that should be applied to every context” [65].

Differentiated Policies for Meaningful Human Control

Other approaches to MHC aim to reconcile the need for differentiated policies with the above ethical and legal constraints on human control. Differentiated policies modulate human control along various autonomy levels for weapons systems. Autonomy levels have been introduced in connection with, say, automated driving, surgical robots, and unmanned commercial ships to discuss technological roadmaps or ethical and legal issues [66,66,68]. A taxonomy of increasing autonomy levels concerning the AWS critical target selection and engagement functions was proposed by Noel Sharkey (and only slightly modified here, with regard to levels 4 and 5) [69••].

  • L1. A human engages with and selects targets and initiates any attack.

  • L2. A program suggests alternative targets, and a human chooses which to attack.

  • L3. A program selects targets, and a human must approve before the attack.

  • L4. A program selects and engages targets but is supervised by a human who retains the power to override its choices and abort the attack.

  • L5: A program selects targets and initiates attack on the basis of the mission goals as defined at the planning/activation stage, without further human involvement.

The main uniform control policies, including those examined in the previous section, are readily mapped onto one of these levels.

L5 basically corresponds to the boxed autonomy policy, whereby MHC is exerted by human commanders at the planning stage of the targeting process only. As noted above, boxed autonomy does not constitute a sufficiently comprehensive and normatively acceptable form of human-machine shared control policy.

L4 basically corresponds to the supervised autonomy policy. The uniform adoption of this level of human control must also be advised against in the light of automation bias risks and increasing marginalization of human oversight. In certain operational conditions, however, it may constitute a normatively acceptable level of human control.

L3 has been seldom discussed in the MHC debate. At this level, control privileges on critical targeting functions are equally distributed between weapons system (target selection) and human operator (target engagement). To the extent that the human deliberative role is limited to approving or rejecting targeting decisions suggested by the machine, this level of human control does not provide adequate bulwarks against the risk of automation bias [70]. In the same way as L4, therefore, it should not be adopted as a general policy.

L1 and L2 correspond to shared control policies where the weapons system’s autonomy is either totally absent (L1) or limited to the role of adviser and decision support system for human deliberation (L2). The adoption of these pervasive forms of human control must also be advised against insofar as some weapons (notably SARMO systems) have long been considered acceptable in warfare operations.

In the light of these difficulties, one might be tempted to conclude that the search for a comprehensive and normatively binding MHC policy should be given up and that the best one can hope for is the exchange of good practices between States about AWS control, in addition to the proper application of national mechanisms to review the legality of weapons [71,71,73]. But alternatives are possible, which salvage the idea of a comprehensive MHC policy, without neglecting the need for differentiated levels of AWS autonomy in special cases. Indeed, the authors of this review have advanced the proposal of a comprehensive MHC policy, which is jointly differentiated and prudential [74, 75].

The prudential character of this policy is embodied into the following default rule: low levels of autonomy L1–L2 should be exerted on all weapons systems and uses thereof, unless the latter are included in a list of exceptions agreed on by the international community of States. The prudential imposition by default of L1 and L2 is aimed at minimizing the risk of breaches of IHL, accountability gaps, or affronts to human dignity, should international consensus be lacking on whether, in relation to certain classes of weapons systems or uses thereof, higher levels of machine autonomy are equally able to grant the fulfillment of genuinely meaningful human control. The differentiated character of this policy is embodied in the possibility of introducing internationally agreed exceptions to the default rule. However, these exceptions should come with the indication of what level is required to ensure that the threefold role of MHC (fail-safe actor, accountability attractor, moral agency enactor) is adequately performed.

In the light of the above analysis, this should be done by taking into account at least the following observations:

  1. 1.

    The L4 human supervision and veto level might be deemed as an acceptable level of control only in case of anti-materiel AWS with exclusively defensive functions (e.g., Phalanx or Iron Dome). In this case, ensuring that human operators have full control over every single targeting decision would pose a serious security risk, which makes the application of L1, L2, and L3 problematic from both military and humanitarian perspectives. The same applies to active protection systems, like Trophy, provided that their use in supervised-autonomy mode is excluded in operational environments involving a high concentration of civilians.

  2. 2.

    L1 and L2 could also be impracticable in relation to certain missions because communication constraints would allow only limited bandwidth. In this case, military considerations should be balanced against humanitarian ones. One might allow for less bandwidth-heavy (L3) control in two cases: deliberate targeting and dynamic targeting in fully structured scenarios, e.g., in high seas. In both hypotheses, indeed, the core targeting decisions have actually been taken by humans at the planning/activation stage. Unlike L4, however, L3 ensures that there is a human on the attacking end who can verify, in order to deny or grant approval, whether there have been changes in the battlespace which may affect the lawfulness of the operation. Looking at existing technologies, L3 might be applied to sentry robots deployed in a fully structured environment, like the South Korean Super aEgis II.

  3. 3.

    The L5 boxed autonomy level should be considered incompatible with the MHC requirement, unless operational space and time frames are so strictly circumscribed to make targeting decisions entirely and reliably traceable to human operators.

Concluding Remarks

Recent advances in autonomous military robotics have raised unprecedented ethical and legal issues. Regrettably, diplomatic discussions at the GGE in Geneva not only have so far fallen short of working out a veritable legal regime on meaningful human control over AWS, but—what is worse—are currently facing a stalemate, which is mainly determined by the opposition of major military powers, including the US and the Russian Federation, to the adoption of any kind of international regulation on the matter.

Our proposal of relinquishing the quest for a one-size-fits-all solution to the MHC issue in favor of a suitably differentiated approach may help sidestep current stumbling blocks. Diplomatic and political discontent about an MHC requirement that is overly restrictive with respect to the limited autonomy of some weapons systems might indeed be mitigated recognizing the possibility of negotiating exceptions to L1–L2 human control, by identifying weapons systems and contexts of use where milder forms of human control will suffice to ensure the fulfillment of the fail-safe, accountability, and moral agency properties whose preservation generally underpins the normative concerns about weapons’ autonomy in targeting critical functions.

In a broader perspective, a differentiated approach to MHC may be of some avail as regards the general issue of human control over intelligent machines operating in ethically and legally sensitive domains, insofar as the MHC language has been recently used about autonomous vehicles [76, 77] and surgical robots [78].