Targeting Third-State Merchant Vessels: Military Objectives and War-Sustaining Objects in the Russo-Ukrainian Armed Conflict

The Russian aggression that precipitated the Russo-Ukrainian armed conflict has long seen both states affirm the validity of forcible economic warfare at sea. Invoking the law of contraband, they have declared and to varying degrees demonstrated their intent and willingness to inspect, intercept and/or divert, capture or destroy private shipping at sea. Notably though, thus far both states had largely, though not exclusively, confined most  of their attacks to territorial waters (pp. 31-32). Ukrainian attacks on third state flagged vessels on November 28th in Turkey’s EEZ marks a departure from that pattern. Ukraine has accepted responsibility for the strikes on two Gambian-flagged oil tankers: the Kairos and Virat, while denying responsibility for the targeting of the Russian flagged Midvolga 2 in the Black Sea or the Panamanian flagged Mersin off Senegal’s coast in the North Atlantic. This was followed by another Ukrainian attack on the tanker Dashan on December 10th, as it transited through Ukraine’s EEZ.

In a recent post on Articles of War, Wolff Heintschel von Heinegg engages with the issue of whether the strikes on the Kairos and Virat were lawful. Adopting a tripartite structure, Heinegg first addresses the applicable legal framework, identifies issues germane to the classification of the vessels, and then focuses on whether they were lawful targets. Central to the final assessment is the interpretation of the definition of a military objective found in Article 52(2) of Additional Protocol I (AP I). Heinegg concludes that the lawfulness of the strikes depends on the status of the doctrine per which a military objective can be interpreted as including not just warfighting or war-supporting objects, but even war-sustaining objects. Gesturing to the Ukrainian targeting of infrastructure crucial for Russian oil exports, he asserts that the lack of state protest or criticism should be considered as a tacit endorsement of this viewpoint.

This post agrees with Heinegg’s concerns about shadow vessels and his observations about vessel classification during hostilities. However, it adds further nuance to Heinegg’s observations about the applicable legal framework and rejects entirely, his arguments on war-sustaining objects constituting valid military objectives in this context.

Vessel Classification 

Heinegg’s assessment of the attacks on Gambian-flagged vessels takes place against the backdrop of sharpening global attention on the issue of shadow shipping – a phenomenon associated with the proliferation of flags of convenience and ships with a tenuous or non-existent genuine link to their flag state. The recent U.S. seizure of the crude oil carrier Skipper off the coast of Venezuela, is but the latest illustration of an issue which has been attracting immense attention in recent years. For purposes of ascertaining a ship’s status, there is now an increasing convergence of views on the legal framework applicable during both peacetime and conflict. It does appear to be increasingly acceptable to look behind the veil of a ship’s registration, so as to ascertain its true nationality and/or enemy character.

The Applicable Legal Framework

Heinegg affirms that the Russo-Ukrainian conflict is an international armed conflict (IAC), and therefore it is the Law of Naval Warfare (LNW), not the law of the sea nor the rules on sanctions enforcement, which provides the relevant legal framework for assessing the attacks on the Gambian-flagged tankers. The LNW is a body of law, which historically authorized belligerents to attack lawful targets everywhere outside neutral waters. Indeed, as Heinegg himself notes, the LNW is characterized by some specific rules which do not apply in land warfare. The most prominent such example is the rules of the LNW relating to economic warfare, which have as their central concern the enemy’s war-sustaining economy.

This distinction between an armed conflict and a war is critical, because this provides the only conceivable basis (para 3.2.3, p.29) for justifying an interpretation of military-objectives as including war-sustaining objects – by way of relying on pre-Charter (para, 275) belligerent rights. However, neither Russia nor Ukraine have declared a state of war. Nor, for purposes of the LNW, are they engaged in war in the material sense, in that, they have not conducted themselves as if the full range of belligerents rights and their associated sub-regimes were operative. Consequently, Ukraine cannot justify the strikes on the Kairos and Virat on the basis of the LNW’s belligerent rights regime, since that regime has not been activated in this conflict. Accordingly, the only pertinent framework is the definition of a military objective found under Article 52(2) of AP I, which has also been incorporated in the LNW, subject to important modifications and exceptions.

War-sustaining objectives and Military Objectives 

Article 52(2) of AP I defines military objectives as 

“those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage“. 

This is a definition which has been adopted verbatim by Rule 40 of the San Remo manual on international law applicable to armed conflicts at sea (SRM) – the manual that is accepted as the authoritative restatement of the LNW.

Per this definition, an oil tanker that is sailing towards enemy controlled territory, intending to supply hostile armed forces, would undoubtedly qualify as a military objective. However, to consider a merchant vessel as a military objective simply because it facilitates trade that contributes to an enemy’s export economy, is a proposition with vanishingly little support amongst legal experts. Article 52(2) AP I clearly requires that the destruction, capture or neutralization of an object purportedly making an effective contribution to military action, offer a definite military advantage. This requirement is understood as requiring a direct and not a remote connection – the latter being precisely what the notion of war-sustaining objects relies upon. This resistance to war-sustaining objects constituting legitimate targets is consistent across treaty text, amongst state’s official positions, in expert restatements generally, and, specifically, in the foundational expert restatement governing the LNW itself as well (the San Remo Manual).

Amongst states, the notion that an individual or an object can become a military objective simply because it plays a ‘war-sustaining’ function (as opposed to a ‘warfighting’ or ‘war-supporting’ function) is a peculiarly American position. Even in the United States, this is a relatively recent (pp. 448-449) development, and one that relies on a historically inaccurate reading of the precedents that supposedly authorize this practice. The only other states whose manuals ostensibly support this stance are the 1983 Belgian law of war manual, Ecuador’s 1989 naval manual, New Zealand’s 1992 military manual and the Indian Navy’s 2022 handbook on the law of maritime operations. The relevant section of the Indian manual is not publicly available, and the Ecuadorian and New Zealand manuals replicate the U.S. language. Notably, New Zealand has since abandoned this position in its 2017 Manual of Armed Forces Law.

Amongst experts, the 1994 SRM on naval warfare, the 2013 HPCR manual on air and missile warfare and the 2017 Tallinn Manual on Cyber Operations, all concur in rejecting the U.S position that the definition of military objectives includes ‘war-sustaining’ objects. The 2025 Newport Manual is the outlier in recognizing the possible legitimacy of the war-sustaining interpretation of military objectives. Indeed, although the Newport Manual notes that States are divided on this issue, even it immediately concedes (pp. 150-151) that the majority of states do not recognize war-sustaining objects as military objectives.

Let us consider how this issue was treated by the authoritative restatement of the LNW: the San Remo manual. As already noted, Rule 40 of the SRM replicates the definition of a military objective found in Article 52(2) of AP I. Rules 60 and 67 then provide domain-specific details about the circumstances in which enemy and neutral merchant vessels can become military objectives. These rules state that breach of a blockade, carriage of contraband, engaging in belligerent acts (laying mines, cutting cables, engaging in search and visit, attacking other vessels), acting as an auxiliary (carrying troops or replenishing warships), assisting the enemy’s intelligence gathering, sailing under an enemy convoy, resisting a valid attempt to visit, search or capture or otherwise making an effective contribution to enemy military action, may be valid grounds for converting a merchant ship into a military objective.

Notably absent from this list is any reference to the notion of the war-sustaining economy. Undoubtedly, this concept is implicit in the reference to blockade and contraband, however, as discussed in the section on the applicable legal framework – the necessary threshold for this (a state of war) is not currently operational. The only possible sub-rule, then, within which the war-sustaining argument could potentially be located, is the reference to “otherwise making an effective contribution to enemy military action.” However, this possibility is immediately excluded when we consider the drafting history of the SRM, where we find that this issue was explicitly flagged, deliberated upon and specifically rejected.

Rule 60(a)-(g) of the SRM lists the activities that may render enemy merchant vessels military objectives. Similarly rule 67(a)-(f) lists the activities that allow for neutral States’ merchant vessels to be attacked. During the drafting of the SRM, there were was a proposed Rules 60(h) and 67(h) as well (pp.147-150 and p.157 and 159). Both of these proposed to render a merchant vessel a valid military objective if it was integrated into the enemy’s war-sustaining effort. However, following a consideration of state practice during the World Wars, the Iran-Iraq conflict, and the Falklands, it was concluded that the rule proposed in clause (h) was not actually a part of customary international law (CIL) and one which would jeopardize the standards set in place by AP I. This is why the decision was made to specifically reject the notion that the integration of a vessel into the enemy’s war-sustaining effort could transform it into a valid military objective.

Over inference from the targeting of dual use objects 

Lastly, there is the issue of how to interpret the muted state response apropos the Ukrainian targeting of infrastructure crucial for Russian oil exports. The debates on this issue track the debates on the targetability of dual use objects, an issue that has been frequently considered in relation to attacks on electrical infrastructure as well. I would like to make three points apropos this issue. 

First, simply because a dual use object qualifies as a military objective, does not mean that the LOAC obligation of proportionality (pp. 2734-2736) ceases to apply. This point is underscored by the ICC’s ongoing investigation into the situation in Ukraine, where the court has issued arrest warrants for conduct involving the directing of attacks directed against civilian objects and for causing excessive incidental civilian harm.

Second, on land, the targeted objects are in territories controlled by the parties to the conflict and (frequently) belong to and principally affect these state parties and their nationals. However, the maritime domain is characterized by the entanglement of structurally embedded neutral and third state rights and interests. To equate these two domains and transpose landward practices, without accounting for the very different contexts and even more elaborate set of applicable laws, is a complex activity that must only be undertaken after the most deliberate consideration.

Third, the muted international response proves little by itself. States are under no obligation to react to each individual strike, and their silence is too weak a basis to suggest that they endorse an interpretation of military objectives that they have repeatedly and explicitly rejected across treaty negotiations and expert processes. This is reflected in the ILC’s work on the identification of CIL as well, which states clearly that state inaction counts as practice only under certain circumstances – it is only deliberate abstention which counts (Conclusion 6, p. 133). Furthermore, failure to react over time may serve as evidence of opinio juris – only provided that states were in a position to react and that this was called for by the circumstances (Conclusion 10 at pp.140-142).

There is currently an ongoing process to update and revise the San Remo manual on international law applicable to armed conflicts at sea. As this process accelerates, states are grappling with how to account for new technological developments, while also reweighing old balances. The recognition of the point that under a paradigm of armed conflict, merely discharging a war-sustaining function does not transform an object into a military objective is one of the most important guardrails of the modern LNW.  Whatever innovations the manual may adopt, it will be critical to not permit the erosion – by omission or expediency, of the hard-won clarity that AP I and the SRM have secured.