Ukraine Symposium – Ukraine’s “Suicide Drone Boats” and International Law

Editors’ Note: As part of our 2023 Year Ahead series, our Board of Advisors member Prof. Dr. Wolff Heintschel von Heinegg presciently identified legal issues concerning maritime drones as a subject to watch this year. It did not take long for the field to respond. Our next two posts, by UK and US naval officers respectively, address the subject in detail. Our compliments to both and to Prof. Dr. von Heinegg.

Aside from the sinking of the Russian cruiser Moskva early in the conflict, no aspect of the naval dimensions of the Russia-Ukraine war has seized the attention of defense analysts more than the events of October 29 near Sevastopol. Using seven unmanned surface vessels (USVs), in concert with unmanned aerial systems (UAS), Ukraine launched a coordinated attack on Russian naval forces, including the minesweeper Ivan Golubets and the new Black Sea Fleet flagship Admiral Makarov.

The strike set off a flurry of commentary from defense experts (see, e.g., here) – although not for the damage it inflicted. Indeed, Russian losses in the attack appear inconsequential. Instead, analysts found compelling the strike’s implications for the future of naval warfare. The world’s navies are now on notice: low-cost, dynamically maneuverable, “suicide” USVs could hold multi-million, even billion-dollar warships at risk. As defense analyst H.I. Sutton observed,

Putting a warhead on a small USV and ramming an enemy warship should have been obvious. But for political reasons, and possibly a lack of imagination in some quarters, that’s not what navies were asking for. That may now change.

As straightforward as it all sounds, lurking behind the basic operational concept Sutton describes are important international law issues. This post will explore those issues. But before doing so, it’s necessary to say more about the new craft attracting so much attention.

Ukraine’s “Shadowy Kamikaze Drone Boats”

The Sevastopol attack wasn’t the first time boats have been packed with explosives and hurled toward an enemy. Sir Francis Drake, the Germans in World War I, and the Houthis all employed the technique with varying success. But the Ukrainian attack stands out for bringing so many elements together: operational nuance, modern technology, number of craft, small size, and above all, extremely low cost and low risk to the attacking force.

Two weeks after the attack, Ukraine lifted the veil on their new weapon in a sleek United24 crowd-sourcing campaign aimed at building more. The “jet-black, very low-slung, jet-drive propelled” craft are 18 feet long and weigh about one ton. According to the United24 website, each USV can travel 500 miles with a top speed of 50 mph. Each can carry a “combat load” of 440 pounds, though it isn’t clear how such a payload reduces range or speed. Further, experts have identified “bow-mounted impact detonation sensors” and speculated that a Space-X Starlink satellite internet antenna adorns the rear. Described as craft that “can be built in almost any garage,” the Ukrainian USVs employ civilian, off-the-shelf components.

Details on exactly how the navigation system works are not disclosed, for obvious reasons. But the basic specifications indicate that the USVs are guided via satellite and inertial navigation and can also be piloted remotely using a video system. It is believed the craft employ a hand-off concept in which the USV is guided to the general area of its target with GPS, at which time a pilot picks up control for the terminal attack run. Indeed, a promotional video on the United24 website shows an operator at a console. Moreover, footage from the Sevastopol attack shows the craft maneuvering to avoid overhead fire. Notably, the USVs have autonomous features. But based on the available information, autonomy extends only to navigation. That is, it appears the craft do not have the controversial capability of identifying, selecting, and engaging targets autonomously.

Finally, how much damage a single USV can inflict on a warship is up for debate. Here, the sinking of Moskva is instructive. Following its demise, analysts questioned professionalism and damage control standards on Russian warships, especially among a largely conscripted force. Others have speculated that Russian ship designers sacrificed survivability in their drive to pack as much firepower aboard vessels as possible. Accordingly, it’s conceivable that a lone USV could sink a Russian warship. More likely, Ukraine will continue to use USVs in swarms – and possibly in numbers exceeding those at Sevastopol – to overwhelm conventional defensive systems.

Legal Status of USVs

Scholars analyzing USVs under international law usually begin with the question of whether these craft qualify as “ships” under the Law of the Sea, and if so, whether they qualify as “warships” (see U.S. Navy/Marine Corps/Coast Guard Commander’s Handbook § 2.2.1 and § 2.3.5; subsequent cites to Commander’s Handbook).

The main advantage flowing from ship status is navigational rights, which allow for free and efficient movement around the world. For example, if USVs qualify as ships, they would enjoy the regime of transit passage through international straits and, in peacetime, innocent passage through foreign territorial seas (U.N. Convention on the Law of the Sea, Articles 38 and 17). In times of war, ships of a belligerent may also enjoy mere passage through a neutral State’s territorial sea, though a neutral State can restrict or even prohibit this passage altogether so long as it does so on a nondiscriminatory basis (Commander’s Handbook § 7.3.4). The neutral State may not, however, suspend passage in international straits or archipelagic sea lanes (at § 7.3.4).

As to warship status, the principal advantage flowing from that designation is belligerent rights. Namely, USVs qualifying as warships would be entitled to attack enemy targets at sea and enforce blockades, among other hostile activities.

Ship Status

The USVs described above are not “ships” for one basic reason: Ukraine has taken no action to flag them as such. As has been observed, “the UNCLOS regime is built on the principle of flag state primacy and subsequent regulation of ships.” Under Article 91 of UNCLOS, each state sets the conditions for the registration of ships in its territory. So, if Ukraine has not flagged their USVs as ships – and to all appearances they have not – then the craft are not ships under the law.

In the future, conditions could evolve to change this result. For example, States could begin flagging their USVs as ships in large numbers. The customary understanding of “ship” might then expand to include most types of USVs. In that circumstance, Ukrainian USVs not flagged would still be sovereign property, but their legal ability to exercise belligerent rights (as discussed below) would be in doubt. As matters stand, though, State practice has not reached a point where this is a concern.

Regardless, ship status and navigation rights matter little to Ukraine at the moment. Most importantly, their USVs have limited range. Currently, they do not have the technical capability to navigate beyond the Turkish Straits (to say nothing of limits imposed by Türkiye under the Montreux Convention). Further, given the geography of the Black Sea, there is no territorial sea or international strait standing between Ukraine and the Russian ships they target. So far, Ukraine’s maritime attacks have been clustered off Ukraine’s coast and near Crimea. There’s every reason to think this situation will persist, leaving Ukraine no reason to concern itself with navigation rights and passage regimes.

Warship Status

If Ukraine had designated their USVs as ships, the next crucial step would have been to designate them as “warships.” Absent this additional step, the USVs could not strike Russian targets without Ukraine’s incurring responsibility for an internationally wrongful act. This is because under international law, warships are the only ships lawfully entitled to exercise belligerent rights (Commander’s Handbook § 2.2.1.1). To be a warship, a ship must belong to the armed forces of a State, bear external marks of nationality, be under the command of a commissioned officer, and be manned by a crew subject to regular armed forces discipline (U.N. Convention on the Law of the Sea, Article 29). Of note, the United States takes the position that unmanned craft may meet these requirements (Commander’s Handbook § 2.3.5). But as noted above, there is no indication Ukraine has flagged their USVs as ships – or warships.

Ukrainian USVs as a Means of Warfare

Given that the Ukrainian USVs are not ships or warships, we now turn to whether the Ukrainian USVs are lawful as a means of warfare. First, some groundwork is necessary. Ukraine and Russia are plainly engaged in an international armed conflict, so the Law of Armed Conflict applies (see, e.g., DoD Law of War Manual § 3.4 and Commander’s Handbook § 5.1.2.1). Further, Ukraine has used its USVs in an “attack” on its Russian adversary. As a result, the USVs clearly qualify as a “means of warfare” under Additional Protocol I to the Geneva Conventions – a treaty to which Ukraine and Russia are both Party. Article 36 of that treaty states,

In the study, development, acquisition, or adoption of a new weapon, means or method of warfare, a High Contracting Party is under an obligation to determine whether its employment would, in some or all circumstances, be prohibited by this Protocol or by any other rule of international law applicable to the High Contracting Party.

Accordingly, Ukraine was obliged to review its USVs for compliance with international law before using them in combat. Whether Ukraine conducted such a review is difficult to know, as many States do not make such reviews public.

The most important question with respect to the legality of the systems is whether the craft are capable of being directed at a specific military objective like a Russian warship (Additional Protocol I, Article 51, § 4(b)-(c)). From what we can tell, the USVs are remotely piloted after a virtual hand-off and maneuvered toward their target using a camera feed with night vision capability. The agility one USV shows in skimming across the water and evading Russian fire in videos, such as this one, is the strongest evidence on this point. It shows that the craft are agile enough to be directed at a specific military objective.

Additionally, as observed by Schmitt and Goddard, a review should also take into account the characteristics of the environment into which the candidate weapons system will be introduced. Here, the war has reduced civilian traffic in the Black Sea between Odessa and Crimea. The unique environment thus further supports the conclusion that USVs would be sufficiently precise to discriminate between lawful military objectives like Russian warships and civilian vessels.

Hague VIII and Torpedoes

A further question is whether the USVs would violate any specific weapons prohibitions under international law. Some, following the approach put forward by Nasu and Letts, will contend that because the USVs are meant for one-way strike missions, they share characteristics with a torpedo. On this view, the USVs must comply with the relevant law for torpedoes found in the 1907 Hague Convention VIII. That treaty, which has become customary international law, prohibits torpedoes that do not become harmless after missing their target (Commander’s Handbook § 9.4; San Remo Manual, Article 79).

Those supporting Hague VIII applicability contend that, like torpedoes, the Ukrainian USVs (a) travel through water, (b) with the goal of striking enemy vessels, and (3) can become a danger to civilian traffic if they miss their target. Thus, interpreting the treaty teleologically based on its object and purpose would bring the Ukrainian craft under Hague VIII. This approach would advance the protective goals of the Law of Armed Conflict and is the stronger position.

On the other hand, those opposing Hague VIII applicability assert that the treaty (a) doesn’t define “torpedo” and (b) only sought to protect against the sort of “dumb” threats contemplated at the time of the treaty in 1907 – that is, basic torpedoes and mines (see Schmitt and Goddard, which discusses the issue, generally). On this view, the treaty wouldn’t touch advanced surface vessels guided by satellite technology and remote operators.

Assuming for the sake of argument that Hague VIII applies, compliance would logically hinge on factors such as range with a combat load, the ability to steer the vessel safely to a place where it can be pulled from the water, or the ability to defuse or detonate the warhead remotely.

Overall, we still don’t know much about the USVs (including, for example, how and from where they were launched). Nevertheless, there is no obvious impediment to their qualifying as lawful weapons under international law.

Looking Ahead

In developing USVs, navies have largely proceeded methodically. They also have prioritized intelligence, surveillance, and reconnaissance missions. But with national survival at stake, necessity compelled Ukraine to employ USVs in a strike role. Though the Sevastopol attack may have done little damage, Russia’s response indicates it takes the new threat seriously. Indeed, the Black Sea Fleet has largely remained in a protective crouch, and Russia struck Ukrainian civilian infrastructure with missiles in the aftermath of the attack.

Whether modern navies will adopt the Ukrainian USV model to complement their existing fleets remains to be seen. Last year, the United States Navy took a step in that direction, announcing an effort to “rebalance the fleet away from exquisite, manpower-intensive platforms toward smaller, less-expensive, yet lethal ones,” as commentators had urged.

The asymmetric benefits of that model cannot be lost on one U.S. partner in particular – Taiwan. Given the roughly 100-mile width of the Taiwan Strait, the USVs employed at Sevastopol appear well-suited to counter a Chinese amphibious invasion. As with the situation in the Black Sea, there would be little need for Taiwan to concern itself with navigation rights and “ship” status. Rather, a key legal challenge for USVs in a Taiwan Strait scenario would concern problems with identifying lawful military objectives. Given the uncertain volume of civilian traffic in that waterway at the time of an invasion, as well as China’s propensity to employ fishing vessels and other civilian craft for military purposes, which qualifies them as military objectives, distinguishing lawful targets from civilian vessels could prove difficult.

“Swarm” tactics, should they be employed, may present another legal challenge. Commentators have for several years highlighted the challenges such tactics will present to a traditional fleet. Swarming relies on “large numbers of relatively low-cost, expendable agents that can be used to overwhelm an adversary.” Importantly, a true swarm acts without a human operator, the basic idea being that the unmanned devices continually coordinate with each other to make the most effective strike. From what is currently known, the USVs employed by Ukraine lack this level of sophistication; human operators likely steered them toward their target. But should similar USVs take on the level of artificial intelligence necessary to carry out a true swarm attack, this may challenge compliance with the principles of distinction, proportionality, and precautions in attack under the law of naval warfare (U.S. Naval Commander’s Handbook §§ 5.3.4, 5.3.3, and 8.31). For instance, would USVs be able to identify a hospital ship in a chaotic environment? Could a swarm attack be suspended or called off if it appeared a civilian ferry was close by and likely to be damaged?

On the other hand, the small size and weapons payload of some USVs may make them the preferred – and, potentially, the required – means of attack in some circumstances. For example, a USV may present less risk to civilian objects near a targeted military objective than, say, a cruise missile (see Schmitt and Goddard, International Law and the Military Use of Unmanned Maritime Systems, page 22). Taiwan’s ability to counter a cross-Strait invasion may very well hinge on its ability to employ weapons systems similar to those Ukraine has employed in the Black Sea – and to extend that model with new software and swarm tactics. Then as now, the legal issues are less likely to be about ship and warship status and more likely to concern the application of traditional the Law of Armed Conflict concepts (such as distinction, and even the law governing torpedoes) to new weapons systems.

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Commander Charles M. Layne is Director of Maritime Operations and a military professor at the Stockton Center for International Law at the U.S. Naval War College.

Photo credit: XOCEAN-XO-450