The naval dimensions of the current international armed conflict between the United States and Israel, on one hand, and Iran, on the other, quickly escalated from an initial focus on sea-to-shore targeting by naval forces to action against Iran Navy and Islamic Revolutionary Guard Corps (IRGC) Navy vessels and units, some of which were in port and others at sea. The sinking of the IRIS Dena south of Sri Lanka and the internment as neutrals of two other Iranian navy vessels and their crews–IRIS Bushehr in Sri Lanka and IRIS Lavan in India–expanded the geographical scope of naval operations. Attacks by Iran against merchant vessels both inside and outside the Gulf (see here) have further broadened the naval dimensions of the conflict to include neutral vessels (“deplored” in UNSC Res. 2817). And the effective closure of the Strait of Hormuz by Iran has drawn global attention due to its impact on the world’s oil supply.
In this series of questions and answers, we explain how the law of naval warfare bears on these and prospective belligerent operations.
Q1: What law applies to naval operations during this conflict?
The two primary bodies of law governing naval operations are the law of naval warfare, which is part of the law of armed conflict, and the law of the sea. The former governs how belligerents must conduct military operations at sea. It includes rules on, for example, which vessels may be attacked; when merchant ships may be visited and searched, captured, destroyed post-capture (as distinct from attack), and attacked; and how naval blockades may be declared and enforced.
The law of naval warfare also incorporates the core rules of the law of armed conflict, such as distinction, proportionality, and precautions in attack. However, the way in which these rules apply at sea is not always the same as on land. For example, targeting at sea is primarily “platform-based;” the platform’s status and/or conduct is generally the starting point for applying the law, and the vessel’s crew shares the targeting liability of their vessel. That said, when naval forces engage in sea-to-shore targeting (e.g., carrier-launched airstrikes, ship- and submarine-launched missile attacks), the rules governing attacks on land apply, not those on targeting at sea. (1907 Hague IX; 1977 AP I, article 49(3)).
It is clear that the law of naval warfare applies in the current situation because the conflict between Iran and those States with which it has engaged in “hostilities” (most significantly, Israel, the United States, the Gulf States, and Jordan) qualifies as an “international armed conflict,” that is, a conflict between States (NWP 1-14M, § 8.1.1; Newport Manual, §§ 1.2, 2.1, ch. 5; San Remo Manual, ¶¶ 1–2). And since the conflict is international, a further key component of the law of naval warfare–the law of neutrality at sea–also applies. Neutrality governs the legal relationship between belligerent States and States not involved in the conflict. For instance, the law of neutrality sets forth the rights and obligations of neutral States regarding activities in neutral territory, such as a neutral territorial sea, and the treatment of neutral shipping and commerce. (NWP 1-14M, ch. 7; Newport Manual, ch. 11; San Remo Manual, ¶¶ 14–18, 118–124).
The law of the sea, which also applies in peacetime, establishes the maritime framework for operations by defining maritime zones, including the territorial sea, international waters, and international straits. This framework is set out in the Law of the Sea Convention (UNCLOS). Although not all belligerents and neutral States are Party to the instrument, its provisions relevant to the conflict are generally accepted as reflecting customary international law binding on all States. Law of the sea rules are particularly important during armed conflict because they determine where belligerent and neutral rights and obligations apply. In this sense, they shape the geography of naval operations. (NWP 1-14M, ch. 2; Newport Manual, § 1.1; San Remo Manual, part. II). It must be emphasized that should the law of naval warfare and the law of the sea rules conflict, the former prevails as lex specialis (the principle that the more specific body of law prevails).
Q2: What categories of ships are subject to attack because they are military objects per se?
During any armed conflict, attacks may target only military objectives, regardless of the domain in which they are carried out. At sea, some vessels are military objectives by nature, while others qualify as military objectives by virtue of their current use, their intended future use, or their activities at the time of the attack. The law of naval warfare sets out how these general rules apply to specific categories of vessels. (Commander’s Handbook on the Law of Naval Operation, NWP 1-14M, §§ 8.1, 8.2.5; Newport Manual, § 8.5; San Remo Manual, ¶¶ 40–41). Note that attacks must be conducted outside neutral waters and may not be carried out if the vessel has unambiguously indicated its intention to surrender (e.g., NWP 1-14M, § 8.6.1).
The United States and Israel have attacked a significant number of Iran Navy and IRGC Navy vessels, including frigates, gunboats, and minelayers. As explained below, doing so was lawful because they qualified as military objectives as such by virtue of their status as warships or auxiliaries, and they were apparently outside of neutral waters at the time of attack.
Warships: Enemy warships are always lawful targets; they are military objectives “by nature.” A warship is a ship belonging to the armed forces of the State, bearing the external markings that distinguish such ships, under the command of a commissioned officer, and manned by a crew that is subject to military discipline. This definition comes from UNCLOS (Article 29) and applies during armed conflict at sea. In fact, the UNCLOS definition was itself drawn from a law of naval warfare instrument (1907 Hague VII, arts. 1-4) that regulates how merchant vessels can be converted into warships, and the 1958 Convention on the High Seas (art. 8(2)), to which the United States is Party.
The category of “warships” includes surface combatants, submarines, and other commissioned naval vessels. Once positively identified, enemy warships may be attacked without prior warning. If there are civilians or civilian objects in the area (for example, an oil rig and its workers), then the attack must also comply with the rule of proportionality. At sea, this means considering whether civilians within the “effects envelope” (for example, on an uninvolved merchant vessel sailing past the targeted warship) and the merchant vessel itself (as a civilian object) will be harmed. In such situations, the attack must also address the requirement to take precautions to minimize harm to those civilian persons and civilian objects. (NWP 1-14M, § 2.2.1, 8.6.1; Newport Manual, §§ 3.2, 8.6.1; San Remo Manual, ¶¶ 13(g), 65-66).
Naval Auxiliaries: Naval auxiliaries may be attacked on the same basis as warships, which means the platform-based targeting rules apply. Naval auxiliaries are ships owned by or under the exclusive control of the State and used for the time being on government non-commercial service in support of the armed forces. They are not warships because they are usually under the command of a civilian master rather than a military officer. Examples include fleet replenishment vessels, intelligence collection ships, troop transports, and military logistics vessels. (NWP 1-14M, § 2.3.1, 8.6.1; Newport Manual, §§ 3.4, 8.6.1; San Remo Manual, ¶¶ 13(h), 65-66). Some auxiliaries are “full-time,” such as vessels of the British Royal Fleet Auxiliary or the United States Naval Ships (USNS). Others may be auxiliaries for the time being, such as merchant vessels temporarily “taken up” from trade and employed by the military in auxiliary roles–like the British-registered Atlantic Conveyor, which Argentina sank during the 1982 Falklands/Malvinas War.
There are, however, some categories of warships, auxiliaries, and merchant vessels–whether enemy or neutral–that are specially protected from attack or capture (see below) so long as they are “innocently employed” in their designated role. These include hospital ships, small coastal fishing vessels, and rescue craft (NWP 1-14M §§ 8.6.3.1, 8.6.3.2; San Remo Manual, ¶¶ 47-52; Newport Manual, § 10.4). To date, there are no indications of any situations involving specially protected vessels arising in this conflict.
Q3: How does the law of naval warfare deal with merchant vessels?
In the law of the sea and law of naval warfare, the term “merchant vessel” denotes any vessel that is not a sovereign immune vessel (UNCLOS, Part II, Section 3, sub-section B; San Remo Manual, ¶ 13(i)). Iran has engaged in a series of attacks on merchant vessels–mainly tankers–in the area of operations. Some of these attacks have occurred inside the territorial seas of Persian Gulf States.
The law of naval warfare permits attacks on merchant vessels in certain limited circumstances (see below), although not in neutral waters. Still, it does so within the context of an escalating set of measures (see, e.g., Newport Manual, § 8.1.1). The first measure is usually “visit and search,” which helps determine, amongst other things, the vessel’s nationality. The next is capture, which can then be “perfected” by condemnation in “prize,” meaning that the ownership of the vessel and possibly its cargo is transferred to the capturing State. Only a “prize court” may lawfully effect such a transfer. If for some reason a captured merchant vessel (the prize) cannot be taken to a capturing State’s port (or that of a co-belligerent) for the next steps, it can (the third measure) be destroyed at sea (generally by sinking), but only after putting the ship’s papers in a place of safety and providing for the safety of the crew and any passengers (NWP 1-14M § 8.6.2.1; San Remo Manual, ¶¶ 151-152; Newport Manual, § 9.13). Finally, if the merchant vessel’s conduct qualifies it as a military objective, it may be attacked.
However, there are important differences in the law that depend upon whether the merchant vessel is enemy or neutral.
Enemy Merchant Vessels: As a general matter, enemy merchant vessels are liable to “capture” outside neutral waters. In the law of naval warfare, flying the flag (or claiming the nationality) of the enemy State is determinative of a merchant vessel’s nationality (e.g., San Remo Manual, ¶ 112). Unlike neutral merchant vessels, there need not have been any specific act or situation to trigger this right of capture; it is available solely based on the vessel’s status as an enemy merchant vessel. As noted, following capture, the vessel may be taken as prize and subjected to prize court adjudication.
Enemy merchant vessels may be “attacked” only if, and for such time, as they qualify as military objectives–for example, by engaging in hostilities (such as laying naval mines), conducting activities otherwise performed by naval auxiliaries, or gathering or transmitting tactical intelligence. They are also subject to attack if they actively resist visit and search, are sailing under a convoy with enemy warships or aircraft (more on this scenario below), or are armed beyond what is reasonably required to defend themselves. But an attack is the last option, available only if capture is not possible. (NWP 1-14M, § 8.6.2; Newport Manual, §§ 8.6.3, 9.4, 9.13; San Remo Manual, ¶¶ 59-61, 135, 138-39).
Neutral Merchant Vessels: By contrast, neutral merchant vessels may not be captured merely because they trade with the enemy. However, the claim of a neutral “flag” is not dispositive and can be displaced. A merchant vessel may be reclassified as an enemy merchant vessel where the situation so indicates, for instance, through enemy ownership or operating under the control of the enemy (San Remo Manual, ¶¶ 113-117).
Neutral merchant vessels suspected of carrying “contraband,” breaching or attempting to breach a lawful blockade, or otherwise rendering “unneutral service” may be stopped, visited, searched, diverted, and captured in accordance with the law of naval warfare. If captured, they must be subjected to prize court adjudication if they are to be condemned into the captor’s ownership. As with enemy merchant vessels, they may only be attacked if they become military objectives or otherwise lose their protection–for example, by actively resisting visit and search, or by clearly acting as naval auxiliaries for the enemy. (NWP 1-14M, §§ 7.4-7.6, 7.10; Newport Manual, §§ 8.6.5, 9.6, 9.9; San Remo Manual, ¶¶ 67, 118–124).
Whether the destruction of a neutral merchant vessel that has engaged in these activities is permissible if it cannot feasibly be captured and sent into a belligerent port is unsettled. The United States, for instance, allows destruction, while the United Kingdom does not (NWP 1-14M, § 7.10.1; UK Law of Armed Conflict Manual, ¶ 13.103). The San Remo Manual states that destruction rather than diversion of both enemy and neutral merchant vessels is available in limited situations (¶¶ 139-140, 151-152).
Q4: If a vessel is subject to attack, what measures, if any, does the law of naval warfare require of the attacking or capturing force?
Enemy warships and naval auxiliaries: Once an enemy warship or naval auxiliary is positively identified as a military objective, there is no requirement of prior warning before attack. (NWP 1-14M, §§ 8.1, 8.6.1; Newport Manual, §§ 8.4–8.5, 8.6.1; San Remo Manual, ¶¶ 40–41, 44–46, 65–66).
Enemy merchant vessels: If an enemy merchant vessel qualifies as a military objective (see above), it may be attacked without warning. However, in the case of an enemy merchant vessel that is not a military objective but is subject to capture based on its status as such, the capturing force is obligated to place the “passengers, crew and ship’s papers in a place of safety” if the vessel is to be destroyed because it is not possible to capture it as prize and diverted to a belligerent port (see above). (NWP 1-14M, § 8.6.2; Newport Manual §§ 8.6.3, 10.4.1.5.2; San Remo Manual, ¶¶ 138–139).
Neutral merchant vessels: Neutral merchant vessels are, as discussed above, subject to capture under certain limited circumstances. If a neutral merchant vessel cannot be effectively captured and diverted to a belligerent port, and is to be destroyed instead, then, so far as circumstances permit, the passengers and crew must be taken to a place of safety, and the ship’s papers must be preserved. (NWP 1-14M, §§ 7.4–7.6, 7.10; Newport Manual, §§ 9.13; San Remo Manual, ¶¶ 67, 118–124, 148–150).
Q5: Can oil carried in neutral merchant vessels be treated as contraband?
Yes, in limited circumstances. As a general matter, oil is susceptible to military use. It may, therefore, in certain circumstances, be treated as “contraband” when carried in neutral merchant vessels (see here and here at § 631). A critical factor is the cargo’s destination. The requirement is that it is destined for enemy-controlled territory. Accordingly, oil exported for commercial purposes cannot qualify as contraband.
When oil does qualify as contraband, belligerent warships may stop, visit, search, and capture a vessel carrying it or suspected of carrying it. The vessel and its cargo may be seized and diverted for adjudication before a prize court. Importantly, carrying contraband is not a basis for attack unless the vessel resists the visit and search. (NWP 1-14M, § 7.4.1; Newport Manual, § 9.6.2; San Remo Manual, ¶¶ 148–150)
Enemy merchant vessels, however, are already liable to capture as prize regardless of their cargo; therefore, the concept of contraband applies only to goods carried on neutral vessels.
Q6: Can tankers carrying export oil be attacked on the basis that they are “war-sustaining” military objectives?
The legality of attacking tankers carrying export oil turns on the interpretation of the term “military objective.” If such tankers are military objectives by virtue of the oil that they are transporting, they are subject to attack, not just capture as an enemy merchant vessel or a neutral merchant vessel carrying contraband to (not from) enemy-controlled territory. There are two views.
The traditional view: Under the traditional view, with which we agree, commercial exports do not become military objectives merely because they generate significant revenue for a belligerent. To qualify as a military objective, an object (the tanker and the oil in it) must make an effective contribution to military action, and its destruction must offer a definite military advantage (ICRC Customary International Humanitarian Law study, rule 8). Ordinary export trade, including oil sold on the open market, lacks a sufficiently direct connection to military action to satisfy this test. (NWP 1-14M, § 8.2.5; Newport Manual, § 8.5.1; San Remo Manual, ¶ 40).
This conclusion is reinforced by the structure of the law of naval warfare, which countenances certain forms of economic warfare. Traditionally, maritime commerce during an international armed conflict has been regulated through visit and search, contraband measures, capture as prize, and blockade. Consistent with that framework, the San Remo Manual, for example, provides that enemy merchant vessels may be attacked only if they qualify as a military objective by transporting troops or military supplies, acting as auxiliaries, or resisting visit and search, etc., but not merely by transporting commercial exports. (¶¶ 59–60, 118–124, 146–150; see also NWP 1-14M, §§ 7.4–7.6, 8.6.2; Newport Manual, §§ 8.6.3, 9.4–9.6).
The “war-sustaining” argument: The broader argument, for which the United States is the primary proponent, holds that objects that sustain the enemy’s ability to continue fighting are military objectives. The paradigmatic contemporary example is oil that is being exported, or that will be, which enables funding the war effort; exports qualify if their destruction would significantly degrade the enemy’s military capability. Applied to maritime trade, this approach would treat certain oil-export tankers, particularly those generating revenue closely tied to financing military operations, as part of the economic infrastructure sustaining hostilities. (NWP 1-14M, § 8.2.5; Newport Manual, § 8.5.1)
Q7: Is the use of naval mines lawful?
Yes. As a general rule, naval mines may be placed in the territorial waters of parties to the conflict and in international waters, so long as they are deployed in accordance with the rules governing mine warfare described below. It is prohibited to lay mines in neutral States’ territorial waters. (NWP 1-14M, §9.2; Newport Manual, § 6.7; San Remo Manual, ¶¶ 80–92).
Belligerents use naval mines, for example, to deny the enemy access to certain areas, provide coastal and harbor defense, enforce a blockade, and attack enemy surface and subsurface warships. The capabilities of mines range from simple contact mines to those with target selection and homing guidance capabilities. They can be armed and detonated by physical contact, acoustic or magnetic signature, or even a change in water pressure as ships pass. Sometimes, mines are “armed,” meaning no safety devices are activated, and they detonate when preset parameters are met. In other cases, mines are “controllable” in this sense, requiring some form of instruction to be armed. (NWP 1-14M, §9.2).
It has been reported that Iran has sown naval mines in the Strait of Hormuz area, which (see below) is certainly belligerent water insofar as Iran’s territorial sea is concerned, and may be belligerent waters with respect to Oman’s territorial sea. That said, if this action results in effectively denying transit passage of neutral merchant vessels through the international strait, the minelaying would be prohibited. Other restrictions also apply, which we discuss below.
Q8: Are there restrictions on the use of naval mines in the law of naval warfare?
Yes. In light of the potential to interfere with neutral shipping and the risk that mines will be used indiscriminately, the law of naval warfare imposes strict restrictions on their use. Most importantly, and like other weapons, naval mines must not be of a nature to strike military and civilian vessels without distinction (indiscriminate attack) and must comply with the rule of proportionality.
The principal treaty specifically addressing naval mines is the 1907 Hague Convention VIII; today, its provisions are generally considered to reflect customary law binding on all States. Under customary international law, the belligerent laying the mines must provide effective warning of the minefield as soon as military conditions allow, for example, through navigational warnings (e.g., NAVTEX/Notices to Mariners). That party must record the mines’ position, for after the cessation of active hostilities, parties must, as soon as possible, remove or render harmless the mines they laid. There are also required technical safeguards: anchored mines must become harmless if they break free, and unanchored mines not attached to the seabed must become harmless within one hour if control is lost.
Although naval mines may be used to channel neutral shipping, they may not be used in a manner that effectively prevents or suspends transit passage for neutral vessels through international straits like the Strait of Hormuz. Additionally, mines may not be laid off the enemy’s coast for the sole purpose of intercepting commercial shipping, but they can be part of a larger naval campaign, such as a properly declared and enforced blockade of a particular port or coastal area. Finally, belligerents are not allowed to seed large areas of international waters and international straits with mines in a way that creates large de facto exclusion zones for all maritime traffic. Instead, areas that are mined have to be reasonably limited and have a concrete military purpose. Neutral vessels should be able to pass through or around them with a reasonable assurance of safety, so far as military necessities permit. (NWP 1-14M, §9.2.3; Newport Manual, § 6.7; San Remo Manual, ¶¶ 80-92).
Q9: Can Iran legally “close” the Strait of Hormuz?
No, with one caveat. Under the customary law of the sea, “transit passage” through neutral waters in a strait used for international navigation (“international straits”) (see also UNCLOS, arts. 37-44) cannot be hampered even during armed conflict (see a recent analysis here). This includes passage by belligerent warships, auxiliaries, and merchant vessels, although the belligerents must not engage in hostilities in neutral waters that are part of the strait, nor use its neutral waters as a sanctuary.
The Strait of Hormuz straddles the territorial seas of Oman and Iran. Neutral merchant vessels are entitled to transit this international strait despite the armed conflict. Vessels of the belligerents may do so through the strait’s Omani waters, but only if Oman is neutral (see below). Obviously, U.S., Israeli, and other belligerent warships, auxiliaries, and merchant vessels that enter the strait’s Iranian waters do not enjoy the right of transit passage. Their warships may lawfully conduct operations against Iran from those waters, but, equally, Iran may target their warships and auxiliaries there. Moreover, their merchant vessels may be subjected to law of naval warfare measures, such as capture and, if they qualify as military objectives, attack.
The analysis depends upon whether Oman is neutral in the conflict. This question hinges on the issue of whether Iranian strikes on Omani territory (see here and here) have brought Oman into the international armed conflict as a belligerent. The Sultan of Oman asserts that his country remains neutral. However, law of armed conflict classification rules hold otherwise, for an international armed conflict exists whenever there are hostilities between two States, even when one or both claim otherwise (1949 Geneva Conventions, common art. 2). Assuming Oman is now a belligerent, the Strait of Hormuz is entirely comprised of belligerent territorial seas. This means that while neutral merchant vessels and warships would continue to enjoy the right of transit passage through it, the Strait would now be a lawful location for exercising belligerent rights, including attack and capture under the law of the sea insofar as belligerent warships, auxiliaries, and merchant vessels of the belligerents are concerned.
Q10: May neutral merchant vessels be escorted?
It has been reported that President Trump has urged other States to assist with merchant vessel protection operations in the area, in particular by escorting merchant vessels through the Strait of Hormuz. For instance, on Truth Social, he stated, “Hopefully China, France, Japan, South Korea, the UK, and others, that are affected by this artificial constraint, will send Ships to the area so that the Hormuz Strait will no longer be a threat by a Nation that has been totally decapitated.” This possibility raises significant law of naval warfare issues regarding convoys and, relatedly, how neutral State actions might cross the threshold into co-belligerency.
The escort of merchant vessels by warships, including through an international strait such as the Strait of Hormuz, is known as “convoying.” The “nationality” of a convoy is primarily set by the escorting warships. This is critical because both enemy and neutral merchant vessels in a convoy under enemy warship escort become liable to attack, as they are considered military objectives due to their participation in the convoy. There is no requirement to try to capture them first. (NWP 1-14M §8.6.2.2, although this only refers to “enemy” merchant vessels; San Remo Manual, ¶ 67(e); Newport Manual, §§ 8.6.3(e), 8.6.5).
By contrast, neutral merchant vessels under convoy of neutral warships are exempt from belligerent visit and search, provided certain guarantees and assurances are given (NWP 1-14M § 7.6; § Manual, Rule 120; Newport Manual, § 9.7.1). However, there are differences in approach to applying the rule. For instance, the U.S. Commander’s Handbook states, “Neutral merchant vessels under convoy of neutral warships of the same nationality are exempt from visit and search” (NWP 1.14M § 7.6). At the same time, the San Remo Manual asserts that the neutral merchant vessel must be under the convoy of “an accompanying neutral warship of the same nationality or a neutral warship of a State with which the flag State of the merchant vessel has concluded an agreement providing for such convoy” (¶ 120(b)). The better view is that the neutral convoy can be escorted by a range of neutral warships (without requiring a special agreement), rather than requiring that neutral merchant vessels receive this protection only if escorted by neutral warships of their own flag (or with a special agreement).
In light of this aspect of the law of naval warfare, President Trump’s call for other States to join the United States in escorting convoys of merchant vessels (likely to be focused on tankers and fertilizer carriers) has two critical consequences.
First, if neutral warships join U.S. warships in convoy escort, the former’s presence would not render the convoy neutral. It would remain an enemy convoy by status insofar as Iran is concerned. This means it would be lawful for Iran to attack both the escorting warships (including the non-U.S. warships) and, as military objectives, the escorted merchant vessels (including any neutral merchant vessels in the convoy).
Second, participation by other States’ warships in U.S.-controlled (and thus, to Iran, enemy) convoys would make those States co-belligerents with the United States, Israel, and other States involved in the international armed conflict. Co-belligerency has already emerged as an issue in the conflict. For instance, the United Kingdom’s defense of regional States and its support for U.S. operations (e.g., allowing the use of airfields) has arguably crossed the threshold. But some of the States that Trump mentions, such as Japan and South Korea, have had no involvement in the conflict. Participation by their warships in U.S.-controlled convoys would render them parties to an international armed conflict, to which the law of naval warfare would apply as described above.
If, by contrast, convoys were structured such that neutral State warships only escorted neutral merchant vessels and no U.S. or other co-belligerent units participated, the convoys would retain neutral status and, accordingly, their prima facie exemption from capture and attack, as well as their right to unhampered (neutral) transit passage through the Strait of Hormuz.
[Editor’s note: Readers may also be interested in Mark Nevitt, Legal and Operational Issues in the Strait of Hormuz: Transit Passage Under Fire]
