The Law of Naval Warfare and the U.S. Capture of Neutral Merchant Vessels: The Case of the Marinera

Since capturing Venezuelan President Nicolás Maduro in early January, the Trump administration has been trying to take control of the country’s oil, an effort that involves seizing oil tankers with connections to Venezuela. To date, the United States has seized seven tankers with links to Venezuela. On Jan. 7, 2026, after a weeks-long pursuit, U.S. forces seized the Russian-flagged tanker Marinera in the North Atlantic. The U.S. military acted pursuant to a federal seizure warrant tied to the vessel’s role in transporting sanctioned Venezuelan oil. Previous articles on Just Security (here and here) examined the seizure under the United Nations Convention on the Law of the Sea (UNCLOS)-based maritime law enforcement regime.

We now turn to an assessment of the operation against a second legal regime – the law of naval warfare (LoNW). Consequently, rather than use the term “seizure,” we switch to “capture,” the LoNW term of art for the action. Since its forces conducted the capture, our analysis will rely heavily on U.S. official sources that set out the LoNW rules for naval operations during armed conflicts. This approach allows us to assess whether the operation passed muster under even the United States’ own interpretation of the law. We will also draw on two highly respected expert manuals that many navies around the world use for operational and legal guidance, the Newport Manual on the Law of Naval Warfare and the San Remo Manual on International Law Applicable to Armed Conflicts at Sea.

In our view, there are four key LoNW questions that the Marinera capture raises, and two peripheral issues. We conclude that the capture was likely inconsistent with U.S. obligations under LoNW.

1) Does LoNW govern the capture?

The threshold question regarding the applicability of LoNW to a “belligerent’s” (party to the conflict) conduct at sea is, as with the law of armed conflict (LOAC) more broadly, whether an international armed conflict (IAC) is underway. Common Article 2 to the four 1949 Geneva Conventions sets forth the universally accepted definition of an IAC: “cases of declared war or of any other armed conflict which may arise between two or more of the High Contracting Parties.” That article also extends such conflicts to occupation, and it is well accepted that the establishment of a blockade can trigger an IAC (see our discussion here).

There is no doubt that, as a matter of law, an IAC between the United States and Venezuela was underway at least from the launch of the Jan. 3 operation to capture Maduro and his wife Celia Flores (see analysis here, here, and here). As has been explained on Just Security, “The intensity of the U.S. operations directed at Venezuela clearly crossed any conceivable threshold necessary to trigger an international armed conflict. To be clear, the operations put the United States and Venezuela in armed conflict as a matter of fact and of law.” And it might be argued that the IAC commenced even earlier, when the United States announced a partial blockade of Venezuela (although our view is that it did not).

We note that the United States has denied the existence of an IAC with Venezuela. For example, U.S. Ambassador to the U.N. Mike Waltz stated during a Jan. 5 Security Council meeting that, “As Secretary [Marco] Rubio has said, there is no war against Venezuela or its people. We are not occupying a country. This was a law enforcement operation in furtherance of lawful indictments that have existed for decades.”

As a matter of practice, what States say about the legal status of their operations is obviously influential in such assessments, but it is not definitive. Common Article 2 confirms that an IAC can exist “even if the state of war is not recognized by one of [the parties].” Instead, whether an armed conflict exists is a question of fact. The ICRC’s Commentary to the article explains,

the determination of the existence of an armed conflict within the meaning of Article 2(1) must be based solely on the prevailing facts demonstrating the de facto existence of hostilities between the belligerents, even without a declaration of war.

Even if none of the Parties recognize the existence of a state of war or of an armed conflict, humanitarian law would still apply provided that an armed conflict is in fact in existence… The fact that a State does not, for political or other reasons, explicitly refer to the existence of an armed conflict within the meaning of Article 2(1) in a particular situation does not prevent it from being legally classified as such” (2017 GC II, Commentary, ¶¶ 233, 235).

Therefore, the only real issue in this case is whether the IAC was still ongoing at the time of the capture. In that regard, Just Security experts pointed out, in an analysis the day before the Marinera’s capture, that,

With massive U.S. forces in the area, the Trump administration’s expressed willingness to use further force as it deems necessary (including repeated threats of doing so if Venezuelan authorities do not “cooperate” with U.S. demands), and its continuing “military ‘quarantine’” of Venezuelan oil tankers, it is uncontestable that the international armed conflict between the United States and Venezuela continues as a matter of law as of writing [Jan. 6].

There is no doubt that an IAC was underway when the capture of the Marinera occurred. Indeed, that operation could, in itself, represent an ongoing use of LoNW authorities and thus provide an (admittedly circular but valid) indication that the IAC was still afoot at that time. To complete the assessment of the incident, we will therefore treat the situation at the time of capture as an ongoing U.S.–Venezuela IAC to which LoNW rules apply.

2) What is Marinera’s LoNW-relevant status and nationality? 

The particular LoNW powers and authorizations available to a party to the conflict in relation to a vessel outside neutral waters generally depend upon its status, nationality, and conduct. There is little doubt that the LoNW “status” of the Marinera is that of a “merchant vessel.” A merchant vessel is a vessel that is: (1) not a warship, nor an auxiliary, nor a State vessel used on non-commercial service; and (2) engaged in commercial activities such as trade, fishing, or passenger transport, or in private activities, such as yachts, pleasure cruisers, and so on (U.S. Navy, Law of Naval Warfare, NWIP 10-2, ¶ 500(b); Newport Manual, § 3.10); San Remo Manual, rule 13(i)).

The second issue is nationality. As previously discussed, the Marinera’s nationality would almost certainly be accepted as Russian under general principles of the law of the sea. The vessel’s apparent Russian registration (flagged) at the time of capture creates a prima facie assumption that she is therefore a neutral merchant vessel (San Remo Manual, rule 113), given that Russia is not a belligerent in the U.S.–Venezuela IAC.

However, during an IAC, a neutral flag, unlike an enemy flag, is not definitive of national character for LoNW purposes. Instead, it allows the belligerent to look behind the flag to ascertain a vessel’s actual national affiliation. In this regard, the U.S. Navy/Marine Corps/Coast Guard’s Commander’s Handbook on the Law of Naval Operations notes that neutral merchant vessels “acquire enemy character” when “1) [o]perating directly under enemy control, orders, charter, employment, or direction” or “2) [r]esisting an attempt to establish identity, including resisting visit and search (§ 7.5.2).

After surveying State practice and opinio juris, as well as scholarly commentary, the experts who drafted the Newport Manual observed that “States widely agree that any merchant ship flying the flag of a neutral State bears enemy character” in the following circumstances (§ 3.10.2).

1. 1. No entitlement under the domestic law of the respective State to fly its flag;
   2. Being under the orders or control of an agent placed on board by the enemy government;
   3. Being in the exclusive employment of the enemy government;
   4. Operating directly under enemy control, orders, charter, employment, or direction;
   5. Owned by enemy nationals or enemy corporations; or
   6. Transfer from an enemy flag to a neutral flag, effected before or after the outbreak of hostilities, and such transfer is made to evade the consequences to which an enemy vessel is exposed.

There is also extensive Prize Law precedent supporting the acquisition of enemy character by neutral merchant vessels during times of armed conflict at sea – see, for example, the United Kingdom case of The Hamborn (19I7).

There is no indication that the Marinera was ever flagged in Venezuela. So, the key nationality issue is whether there is anything behind the claim of Russian registration that would indicate actual Venezuelan control, thereby rendering it an enemy merchant vessel under the LoNW. This would be a significant fact in the Marinera capture because vessels having enemy character are subject to capture based on that status alone (DoD Law of War Manual, § 13.5.1; San Remo Manual, rule 135; Newport Manual, § 9.4).

No open source reporting suggests that any of the aforementioned grounds are satisfied in the case of the Marinera. Consequently, at least in LoNW terms, the tanker appears to have enjoyed the status of a neutral merchant vessel at the time of capture. This is a key finding regarding the vessel’s liability to capture, as LoNW imposes strict limits on the right of a belligerent party to capture a neutral merchant vessel.

3) Was there a LoNW basis for capturing the Marinera?

The Navy’s Law of Naval Warfare (NWIP 10-2) sets forth the U.S. position on conduct that makes a neutral vessel liable to capture beyond neutral waters, one generally accepted by other States and LoNW experts (§ 503(d); see also Manual, § 9.6; San Remo Manual, rule 146). It provides that the following actions justify capture by a belligerent’s warship.

1. 1. Carrying contraband;
   2. Breaking, or attempting to break, blockade;
   3. Carrying personnel in the military or public service of an enemy;
   4. Transmitting information in the interest of an enemy;
   5. Avoiding an attempt to establish identity, including visit and search;
   6. Presenting irregular or fraudulent papers; lacking necessary papers; destroying, defacing, or concealing papers;
   7. Violating regulations established by a belligerent within the immediate area of naval operations.

Once captured by gaining control of the neutral merchant vessel, the vessel becomes “prize” and subject to adjudication by a prize court in the belligerent State, where it may be “condemned” NWIP 10-2, ch. 9). Upon condemnation, title transfers to the capturing State.

We can quickly dispense with grounds 3, 4, 6, and 7 on the basis that there is no indication whatsoever in open-source material of their relevance in the Marinera case. This leaves the remaining three.

Contraband: Contraband, which may be absolute or conditional, “consists of goods that are destined for an enemy of a belligerent, and that may be susceptible to use in armed conflict (Department of Defense, Law of War Manual, § 15.12.1, emphasis added; see also San Remo Manual, rule 148). Absolute contraband consists of goods that are military in character, like weapons or ammunition. Conditional contraband, by contrast, refers to items that can be used for both military and civilian purposes. During an IAC, belligerents may declare “contraband lists” designating items it considers absolute or conditional contraband (§ 15.12.1.3).

Despite the focus on oil shipments in the U.S. operations, there is no effective LoNW U.S. contraband list in place. And more importantly, since the Marinera was not bound for Venezuela, the rule allowing capture based on carrying contraband does not apply in the first place.

Blockade: As we explained in greater detail in an earlier Just Security piece, the U.S. effort to block oil shipments from Venezuela does not constitute a valid LoNW blockade (DoD Law of War Manual, § 7.7.2). There was no required notification of the establishment of a blockade by the United States; it does not encompass all imports and exports (except humanitarian goods); and it is not applied impartially to all neutral vessels. There being no blockade as a matter of law for the Marinera to breach, capture on this basis would not have been lawful.

Avoiding visit and search: Under LoNW, belligerent warships are entitled to conduct a “visit and search” of a merchant vessel that they suspect of being subject to capture, so long as it does not occur in territorial waters. The DoD Law of War Manual provides four non-exhaustive examples of situations in which such a visit and search is appropriate (¶ 15.13.1; see also Newport Manual, § 9.9; San Remo Manual, rule 119).

1. 1. ascertaining the character of the vessel or aircraft and nationality (including assessing whether a vessel or aircraft that is flagged to a neutral State has acquired enemy character by engaging in service to the enemy;
   2. verifying whether it conveys contraband cargo;
   3. verifying whether it has committed a breach of blockade; or
   4. verifying whether the vessel or aircraft has committed another violation of neutrality making it liable to capture.

An argument could be made that: (1) the United States indicated a desire for an ostensibly neutral merchant vessel (Marinera) to stop and submit to visit and search in order to ascertain its relationship, if any, to the IAC; (2) the vessel refused to stop and/or accede to the visit; and (3) U.S. warships were then authorized to pursue and carry out the visit, search and capture. However, this argument fails on the facts.

The right of visit and search of an apparently neutral merchant vessel is designed to ascertain two things. The first is nationality. As noted, if the vessel is determined to be of enemy character, it may be captured. Second, if the vessel is confirmed as neutral, the visiting warship is entitled to ascertain whether there is any cause for suspicion that it is liable to capture. If the vessel’s documents are in order, its neutral nationality is confirmed, and there is no indication of a cargo or other activity that would render it liable to capture, then the basis for the visit and search is at an end. The boarding team must disembark and allow the neutral merchant vessel to continue its voyage. In the Marinera case, the vessel’s clearly neutral status (i.e., that it is not Venezuelan-flagged) and the lack of any relevant basis for suspicion of liability to capture would have precluded capture following the visit and search.

It must be emphasized that the Marinera’s failure to heave to and submit to visit and search when the first attempt to visit was made did not in itself create grounds for subsequent capture. This is because the liability to capture of a neutral merchant vessel–which clearly and deliberately refuses to stop, or clearly and intentionally resists visit and search–must be paired with a concomitant belief “on reasonable grounds” that she is “carrying contraband or breaching a blockade” (San Remo Manual, rule 67(a)). As noted, that was not the case here.

4) Capture based on a right to attack?

It might be claimed that the neutral tanker was liable to capture on the basis that she qualified as a military objective. The DoD Law of War Manual defines a military objective as an object that “somehow makes an effective contribution to military action” and “attacking, capturing, or neutralizing the object, in the circumstances, offers a definite military advantage” (§ 5.6.5). Military objectives, whether on land, sea, in the air, or in space, are subject to attack (§ 5.5).

There was a reported “theory” that “there may be something of value to Moscow within the [Marinera]. While it is empty of oil, the route it previously took between Iran and Venezuela is suspected of being a path for illicit trade, including for weapons.” However, in the absence of any credible evidence that she was transporting weapons or other war material, we cannot assess this claim.

If there is no other cargo of LoNW relevance, then any claim as to contribution would rest, in our view, on two problematic and one contested interpretations of the applicable law. First, reports indicate that the vessel was not carrying Venezuelan oil (or other relevant cargo) when captured. Footage of the vessel appears to confirm that she was in ballast (not loaded with an oil cargo) at the time of capture. So, assuming solely for the sake of analysis that oil exports might be a category of effective contribution to the enemy’s war-fighting capability (for example, through financing arms purchases), this argument would still fail because the vessel was not at the time carrying any oil.

The second problematic interpretation concerns timing: It is well-accepted in LOAC that civilians enjoy protection from attack “unless and for such time” as they directly participate in hostilities. The question is, when does the period of direct participation begin and end? The United States is of the view that “persons who are assessed to be engaged in a pattern of taking a direct part in hostilities do not regain protection from being made the object of attack in the time period between instances of taking a direct part in hostilities” (DoD Law of War Manual, § 5.8.4.2). This is much broader than the position taken, for instance, by the ICRC in its Interpretive Guidance on direct participation (page 70). The analogy to the U.S. approach in the maritime context would be a merchant vessel that repeatedly ships items that make an effective contribution to military action.

However, even if the authority to capture a neutral merchant vessel based on contribution to the enemy’s military activities (1) did extend to oil export, and (2) generates a lingering authority to capture even at a later time when the vessel is not engaged in the activity, the obstacle is that the Marinera had not done so at any time since the start of the armed conflict. Indeed, reports indicate that, while she was en route to take on Venezuelan oil in December 2025, the U.S. operation led her to flee before she could do so. It would be drawing an extraordinarily long bow to argue that conduct prior to the commencement of the IAC, especially unsuccessful conduct, nevertheless rendered the tanker liable to capture after the IAC had commenced.

The third issue – more contested than problematic – is that of “war sustaining” military objects (in a LoNW context, see Newport Manual, § 8.5.1). The United States is of the view that “vessels associated with or in the service of a belligerent State will be military objects by their nature, purpose, use, war-sustaining, or war-supporting roles” (Navy/Marine Corps/Coast Guard Commander’s Handbook on the Law of Naval Operations, NWP 1-14M, § 5.3.4. War-sustaining commerce is that which “indirectly but effectively supports and sustains the belligerents’ warfighting capability” (§ 7.4).

The argument would be that oil exports allow Venezuela to sustain its military activities against the United States, and, therefore, neutral tankers facilitating this trade become military objectives. Because they are military objectives, the LoNW would accordingly allow them to be captured (or attacked). As the Newport Manual notes, however, “States are divided on whether civilian objects that contribute to the enemy’s war-sustaining effort—as distinguished from its war-fighting effort—are legitimate military objectives” (§ 8.5.1.1). We disagree with the war-sustaining interpretation. But even if we did accept it, the LoNW argument around “effective contribution” would nevertheless fail as a consequence of either, or both, of the other “problematic” interpretive issues noted above.

Other material LoNW issues

There are two other interesting LoNW issues the Marinera capture raises. The first is the question of neutral merchant vessels under convoy of neutral warships. Neutral merchant vessels are exempt from the belligerent right of visit and search if they are sailing under convoy of the neutral State’s warships (DoD Law of War Manual, § 15.134.2; San Remo Manual, rule 120). In such a case, the commander of the neutral convoy may be required by a commander of the intercepting warship to confirm the neutral character of the merchant vessel and provide, in writing, information as to cargo that the intercepting warship would have obtained through visit and search (DoD Law of War Manual, § 15.13.2.4).

There were claims of Russian warships in the vicinity of the Marinera. However, there was no assertion that she was under their convoy escort, only that they were nearby. So, this exemption from visit and search is not relevant to a LoNW analysis of the operation.

The second issue concerns support for the operation from the United Kingdom. The BBC reported that “RAF surveillance aircraft and a Royal Navy support ship, the RFA Tideforce, took part in the operation.” Assuming, as we do, that the United States and Venezuela were engaged in an IAC at the time of the pursuit and capture of Marinera, the U.S. capture of the neutral merchant vessel was almost certainly a violation of the LoNW. The seizure was also likely unlawful under the law of the sea, as was explained in the previous article.

Thus, assuming, as we do, that the seizure/capture was unlawful under either legal regime, the United Kingdom would bear responsibility for its contribution to the operation based on “aiding and assisting” an “internationally wrongful act” of another State (Articles on State Responsibility, art. 16). Such responsibility requires that the assisting State know of the circumstances of the wrongful act and that the act would be unlawful had the assisting State carried it out. These requirements were likely met in this case.

Concluding thoughts 

The capture of the Marinera was, as a matter of LoNW, likely unlawful. Although there is, in theory, a narrow path to liability via the “effective contribution” element, an argument along those lines would almost certainly fail given the interpretive hurdles we have laid out. Indeed, if the United States were applying the LoNW, which it should be, the capture would need to be adjudicated before a Prize Court. However, there remains no indication in U.S. statements that the capture has been considered a LoNW-governed operation, which leaves the capture/seizure, as we have noted previously, in very rough seas as a matter of law.

– Rob McLaughlin, Conor McLaughlin and Michael Schmitt, Published courtesy of Just Security.