2.1 Targeting

International humanitarian law generally permits the use of force against objects that ‘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’, according to Paragraph 40 of the San Remo Manual ^{Footnote 14} . Paragraph 67 specifically concerns ‘[m]erchant vessels flying the flag of neutral States’ and lists a variety of instances where they can be targeted. This includes situations where ships ‘are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search or capture’, or where ships ‘otherwise make an effective contribution to the enemy’s military action’. These rules have no geographical limitation and apply in all maritime areas.

The International Law Association’s Helsinki Principles on the Law of Maritime Neutrality (1998) regulate the same question in Paragraph 5.1.2, which largely overlaps with the provisions of the San Remo Manual. Subparagraph 3 allows attacking of neutral ships that ‘are believed on reasonable grounds to be carrying contraband or breaching a blockade, and after prior warning they intentionally and clearly refuse to stop, or intentionally and clearly resist visit, search, capture or diversion’. Subparagraph 4 lists specific situations where neutral merchant ships can be attacked, all of which focus on their connection to the enemy.

The San Remo Manual is a non-binding document which was adopted by the International Institute of Humanitarian Law, an Italian non-profit organization, in 1994. The Manual was written by diplomats and academics, although ‘States were not officially represented’.Footnote ¹⁵ It has been said to have ‘had a tremendous influence on the development of the law of armed conflict at sea’.Footnote ¹⁶ The Helsinki Principles have been said to be ‘widely accepted as being declaratory of the present law’.Footnote ¹⁷ In conclusion it seems safe to assume that international humanitarian law give states a right to use force against neutral merchant ships outside territorial waters in the situations listed in the San Remo Manual or the Helsinki Principles.

2.2 Blockades

The San Remo Manual and the Helsinki Principles both contain provisions that permit the use of force against neutral ships that are ‘believed on reasonable grounds to be … breaching a blockade’. Blockades are ‘a belligerent operation to prevent vessels and/or aircraft of all nations … from entering or exiting specified ports, airports, or coastal areas belonging to, occupied by, or under the control of an enemy nation’.Footnote ¹⁸ Blockades are indiscriminate, in that they must be ‘enforced against every vessel of every nation’.Footnote ¹⁹

The term ‘blockade’ is sometimes used in relation to actions that are not covered by this definition, such as the 2017 decision of Bahrain, the United Arab Emirates, and Yemen to close their own borders with Qatar. These states were not blocking Qatar’s maritime territories from ships and aircraft from third states, and the action was therefore not a blockade under international law, even though Qatar used that term in diplomatic correspondence.Footnote ²⁰

The Paris Declaration Respecting Maritime Law was adopted by 55 states in 1856. While it was called a ‘declaration’, according to its wording it was ‘binding’ on the states parties. The Declaration mentions blockades in Section 4, and states that they ‘in order to be binding, must be effective-that is to say, maintained by a force sufficient really to prevent access to the coast of the enemy’. This means that the states that were parties to declaration recognized blockades as legal but at the same time subject to an effectiveness requirement.

The London Declaration concerning the Laws of Naval War was signed by ten states in 1909 but never came into force. Chapter I (Article 1-21) deals with blockades. It includes a variety of conditions for the legality of a blockade. Articles 14 and 17 mention ‘neutral’ vessels, thus envisaging that a blockade may be enforced against them. Article 1 says that ‘[a] blockade must not extend beyond the ports and coasts belonging to or occupied by the enemy’, which seems to mean that blockades could not be enforced as far off the coast as in exclusive economic zones or on the high seas.

Paragraphs 93–104 of the San Remo Manual cover blockades. The San Remo Manual does not explicitly say that blockades can be established and enforced outside territorial waters, but Paragraph 96 says that ‘[t]he force maintaining the blockade may be stationed at a distance determined by military requirements’.

The Helsinki Principles also attempt to sum up the law of blockades. Section 5.2.10 says that blockades are ‘a legitimate method of naval warfare’ and that ‘[n]eutral vessels believed on reasonable and probable grounds to be breaching a blockade may be stopped and captured’. The Helsinki Principles do not say anything about where, i.e., in which maritime areas, blockades may be established.

In short, under international humanitarian law, states seem to be permitted to establish and enforce blockades against neutral ships, probably outside territorial waters.Footnote ²¹ During the period when the Paris and London declarations were drafted and the law of blockade was developed, the generally agreed limit for the territorial sea was 3 nautical miles rather than the 12-mile limit found in UNCLOS.Footnote ²² A discussion of the current law of blockade must be based on the current extent of the relevant maritime zones.

2.3 Visit and search

The right to visit and search is a ‘right to warships of belligerent States to visit and search foreign merchant ships in order to ensure that they are not carrying contraband to the enemy’.Footnote ²³ This right is ‘generally recognised as reflecting customary international law’.Footnote ²⁴

This right is covered by the San Remo Manual, which says that ‘neutral ships’ can be subjected to visit and search outside ‘neutral waters’ if ‘there are reasonable grounds for suspecting that they are subject to capture’ (paragraph 118).Footnote ²⁵ Paragraph 14 of the San Remo Manual defines ‘neutral waters’ as ‘the internal waters, territorial sea, and, where applicable, the archipelagic waters, of neutral States’, which means that the rule in paragraph 118 applies to exclusive economic zones and the high seas.

Paragraph 146 says when such ships are ‘subject to capture’. It refers back to Paragraph 67, meaning that a ship that can be targeted can also be captured. Paragraph 146 adds some additional situations, where neutral ships make a variety of contributions to the enemy’s war effort. The grounds for capture are ‘carrying contraband’, ferrying enemy armed forces, being controlled by the enemy, falsifying or tampering with documents, ‘violating regulations established by a belligerent within the immediate area of naval operations’, and ‘breaching or attempting to breach a blockade’. Except for the law of blockades, which is a separate question (discussed in Section 2.2 above), and the regulation of ‘the immediate area of naval operations’, these grounds are linked to helping the enemy or being suspected of doing so. Thus, according to the San Remo Manual, exercising the right to visit and search outside the immediate area of naval action will usually require a specific suspicion of a link between the neutral ship and the enemy.

The right to visit and search is also found in the Helsinki Principles. Paragraph 5.2.1 says that ‘belligerent warships have a right to visit and search vis-a-vis neutral commercial ships in order to ascertain the character and destination of their cargo’. Unlike in the San Remo Manual, this is not predicated on any suspicion against the specific ship.

The conclusion is that international humanitarian law contains a right to visit and search. The content of this right is not entirely certain. The San Remo Manuel is the most authoritative source, and it makes the right dependant on a suspicion that a specific ship has some link to the enemy. In the rest of this article, visit and search operations predicated on a specific suspicion against an individual ship will be called ‘discriminate’, while the opposite will be called ‘indiscriminate’.

2.4 Exclusion zones

states may declare that neutral ships will be liable to attack if they enter a designed maritime zone. The establishment of such a zone may be seen as a threat of force,Footnote ²⁶ while its enforcement will generally involve the use of force. The legality of such ‘exclusion zones’ has been debated throughout the history of modern humanitarian law.Footnote ²⁷ Paragraph 105 of the San Remo Manual states that ‘[a] belligerent cannot be absolved of its duties under international humanitarian law by establishing zones’. Moreover, according to Paragraph 106(a), ‘the same body of law applies both inside and outside the zone’. Thus, the regular rules of humanitarian law apply regardless of whether a state establishes an exclusion zone.Footnote ²⁸ This also means that ‘international law does not legitimize attack upon a vessel solely as function of that vessel having entered a predesignated exclusion zone’.Footnote ²⁹ state military manuals have followed the San Remo Manual’s lead.Footnote ³⁰ Older attempts at codification have had varying rules,Footnote ³¹ but they have become less relevant with the adoption of the San Remo Manual.

In short, under international humanitarian law, exclusion zones do not by themselves give any right to use force against neutral ships outside territorial waters. Such rights must instead be found in other parts of humanitarian law, including the right to visit and search or to enforce a blockade.

3.1 The prohibition of the use of force applies to ships

The previous sections have shown that international humanitarian law permits the use of force against neutral ships outside territorial waters in certain situations. The argument in the following sections is that the prohibition of the use of force applies to ships and applies concurrently with international humanitarian law. An action must comply with both sets of rules in order to be legal. The prohibition of the use of force has an exception for self-defence, but the right to self-defence does not give a right to direct the use of force against third states. This represents a challenge to the legality of blockades outside territorial waters and indiscriminate visit and search operations.

The first premise of the argument is that the prohibition of the use of force applies to ships. This means that ships are protected from the use of force by foreign states. The wording of the UN Charter (Article 2(4)) does not specify whether it applies to ships. Article 42 is an exception from Article 2(4). Read in conjunction with Article 41, Article 42 must be rest on the assumption that a ‘blockade’ can be a measure ‘involving the use of armed force’. That must build on an assumption that Article 2(4) applies to ships.Footnote ³²

UNCLOS Article 88 says that ‘the high seas shall be reserved for peaceful purposes’, and this is usually taken to mean that the general international law rules on the use of force apply at sea. ^{Footnote 33} UNCLOS Article 301 obliges states to respect the prohibition of the use of force when ‘exercising their rights and performing their duties under’ UNCLOS, which also seems to build on an assumption that the prohibition applies to ships in the first place.Footnote ³⁴

The UN General Assembly’s Definition of Aggression (1974) includes ‘blockade’ in Article 3(c) and attacks ‘sea or air forces, or marine and air fleets’ in Article 3(d) as forms of ‘aggression’.Footnote ³⁵ The North Atlantic Treaty (Article 6(1)) includes civilian vessels as potential targets of armed attacks. ^{Footnote 36} The ICJ applied the prohibition of the use of force to oil platforms in the Oil Platforms case, and was open to applying it to an individual ship. ^{Footnote 37} The arbitral tribunal in Guyana v. Suriname applied the prohibition of the threat of force to an oil platform and a ship.Footnote ³⁸ Judge Gao wrote in his individual opinion in the ITLOS’ Ukrainian naval vessels cases that ‘the firing of target shots against a naval vessel is therefore tantamount to use of force against the sovereignty of the State whose flag that vessel flies’.Footnote ³⁹ All this supports the view that the prohibition of the use of force is not limited to land territory.

Writers generally agree that the prohibition of the use of force applies to ships.Footnote ⁴⁰ The flag state will be the target of the use of force.Footnote ⁴¹ Guilfoyle states simply that ‘an interdiction not otherwise authorized by international law would be prohibited as involving a threat or use of force’.Footnote ⁴² Guilfoyle rightly rejects the alternative view that ‘boarding and seizing a vessel does not violate the prohibition on the use of force in Article 2(4) of the UN Charter, as the vessel is not relevantly part of the flag State’s territory’.Footnote ⁴³ In Oil Platforms the ICJ stated that an attack on a vessel was ‘not in itself to be equated with an attack on’ the flag state because the ship was not flying the relevant flag.Footnote ⁴⁴ The implication is that if it had been flying the flag, there would have been attack on the flag state.

Some uses of force may involve a coastal state as well as a flag state. If a state uses force against a neutral merchant ship in another state’s exclusive economic zone, this does not constitute a use of force against the coastal state, only against the ship’s flag state. By contrast, a use of force against a ship in a foreign state’s territorial waters is a use of force against that state regardless of the ship’s flag,Footnote ⁴⁵ but that is not at issue in this article.

Some naval actions constitute law enforcement and fall outside UN Charter Article 2(4).Footnote ⁴⁶ For example, the arbitral tribunal in Guyana v. Suriname stated that ‘in international law force may be used in law enforcement activities provided that such force is unavoidable, reasonable and necessary’.Footnote ⁴⁷ The ICJ’s decision in the Fisheries Jurisdiction case between Spain and Canada may be taken to support the same point. Spain had argued that Canada’s actions when ‘arresting’ a Spanish ship and ‘harassing’ others ‘contravenes the provisions of the Charter’.Footnote ⁴⁸ The Court held that Canada’s actions ‘falls within the ambit of what is commonly understood as enforcement of conservation and management measures’, without mentioning Article 2(4).Footnote ⁴⁹ This may have implied that some measure of force may be used against ships without being subject to Article 2(4),Footnote ⁵⁰ but the Court is not clear on this point. The ITLOS, in M/V Saiga, stated that:

The Tribunal did not mention UN Charter Articles 2(4) and 51, which may mean that the tribunal applied a separate rule. A Guinean patrol boat had opened fire on and boarded a Saint Vincent and the Grenadines tanker ship, damaging the ship and injuring two crew members.Footnote ⁵² The Tribunal concluded that this constituted ‘excessive force’ and was contrary to ‘international law’.Footnote ⁵³ The ICJ, in Corfu Channel, denounced the United Kingdom’s minesweeping operation in Albanian waters ‘as the manifestation of a policy of force’, but without mentioning the prohibition of the use of force or UN Charter Article 2(4).Footnote ⁵⁴ The failure to mention these rules could mean that the Court did not believe that the United Kingdom’s actions violated the prohibition, but the judgment is not clear on this point.Footnote ⁵⁵

Whether a state’s actions within its own maritime area is considered law enforcement or a use of force should depend on ‘a case-by-case assessment’.Footnote ⁵⁶ Relevant factors include ‘political context’ (especially the ‘reaction of the victim state’), ‘intensity’, recurrence, the involvement of either police or military, and ‘the level of decision making’.Footnote ⁵⁷ The tribunal in Guyana v. Suriname concluded that Suriname’s actions, where warships ordered an oil rig and drill ship to leave a disputed maritime area within 12 hours and followed them during the departure, ‘seemed more akin to a threat of military action rather than a mere law enforcement activity’.Footnote ⁵⁸ This suggests a relatively low threshold for classifying naval actions as use of force rather than law enforcement.Footnote ⁵⁹

Outside a state’s own maritime areas, UNCLOS provides a number of grounds for law enforcement. When there is no ‘possible nexus’ to any of these grounds, the interception of merchant ships should ‘normally’ be seen as a use of force.Footnote ⁶⁰

In any case, even if a specific visit and search operation or a search conducted as part of a blockade is seen merely as law enforcement, these actions must be backed up by the threat or use of proper force if the target resists.Footnote ⁶¹ This will bring Article 2(4) into play eventually.Footnote ⁶² The interceptions of neutral merchant ships covered by this article will therefore be illegal unless they can be justified by one of the exceptions to the prohibition of the use of force, which are examined further in Section 3.3.

3.2 The prohibition of the use of force and humanitarian law apply concurrently

This section presents the argument that the prohibition of the use of force applies to ships concurrently with international humanitarian law. Blockades were mentioned in the Paris Declaration and the London Declaration, as noted in Section 2.2, but these were drafted before the adoption of the UN Charter. The Charter’s prohibition of the use of force outlawed many practices that had previously been seen as a normal exercise of power in international politics. That a certain practice was legal under international humanitarian law before the adoption of the prohibition of the use of force is not in itself an argument in favour of its post-UN Charter legality.

International humanitarian law and the prohibition of the use force are distinct areas of international law, and they must be distinguished when assessing the legality of naval blockades and other military actions. An action that complies with the prohibition of the use of force may nonetheless violate international humanitarian law, and vice versa. The two sets of rules are aimed at somewhat different facts. The prohibition of the use of force covers mainly the initiation of military conflict, while international humanitarian law governs each individual military action taken as part of a military conflict. However, the prohibition of the use of force applies to limited military operations as well as large-scale ones. Therefore, the two sets of rules overlap, in the sense that they apply concurrently to some of the same actions. An illustration is the 1998 US bombing of the Al-Shifa pharmaceutical factory in Sudan. This violated the prohibition of the use of force, since there had been no ‘armed attack’ by Sudan against the US, the UN Security Council had not authorized the bombing, and Sudan did not consent to it. The bombing probably also violated international humanitarian law, since civilians were killed and injured, and the factory turned out not to have any military function. If there had been, for example, a foregoing ‘armed attack’ from Sudan, the bombing may not have violated the prohibition of the use of force (but it could still have violated international humanitarian law). If the factory had been a military production facility, the bombing may not have violated international humanitarian law (but it could still have violated the prohibition of the use of force). If the US had instead launched a full-scale invasion of Sudan, the invasion as such would have to be judged according to the prohibition of the use of force, while the specific military actions that made up the invasion would have to be judged according to international humanitarian law. While the two sets of rules may in this sense overlap and interact, they do not merge.

When discussing the relationship between the prohibition of the use of force and international humanitarian law, the San Remo Manual (1994) and the Helsinki Principles (1998) are more interesting than the Paris Declaration and London Declaration by virtue of having been drafted after 1945.

The substantive provisions of the San Remo Manual and the Helsinki Principles concern humanitarian law. The San Remo Manual seems to assume that its provisions are subject to limitations imposed by the prohibition of the use of force. It states that a ‘majority of participants’ thought that ‘the rights of belligerents are affected by the restraints of the law of self-defence and that this will affect the rights of belligerents to make full use of all the methods of naval warfare that the traditional law automatically allowed’.Footnote ⁶³ This supports the argument that the two sets of rules apply concurrently and must be analysed separately.Footnote ⁶⁴ The provisions of the San Remo Manual does not help determine what is legal under the prohibition of the use of force.

A similar recognition is found in the UK Military Manual, which states that ‘the conduct of armed conflict at sea is subject to the limitations imposed by the UN Charter on all use of force’.Footnote ⁶⁵

The Helsinki Principles Section 1.2 say that the Principles shall not ‘be construed as implying any limitation upon the powers of the Security Council’ or ‘as denying the inherent right of individual or collective self-defence’ in the UN Charter. In other words, the Principles are not meant to limit any of the rules in the UN Charter, but this article is concerned with the opposite question, whether the UN Charter limits the humanitarian law rights codified in the Principles. The Principles are silent on this point.

The state practice referred to in Section 4.3 below shows states protesting against uses of force against neutral ships outside territorial waters. That is consistent with the view that the prohibition of the use of force applies at sea concurrently with international humanitarian law.

Some writers have presented arguments that in effect mean that the prohibition of the use of force does not apply alongside international humanitarian law outside territorial waters. Sanger argues that ‘[i]f the Gaza blockade is unlawful, the interception of the Gaza Freedom Flotilla vessels … may also amount to an unlawful use of force, prohibited by Article 2(4) of the UN Charter’.Footnote ⁶⁶ The better view is that a blockade may violate the UN Charter regardless of whether it is ‘lawful’ under international humanitarian law. Drew writes that ‘many conflicts are conducted outside of the ambit of the Charter, without direction or involvement of the Security Council’ and that ‘[i]n these cases, the traditional jus in bello, of which the law of neutrality is a part, applies’.Footnote ⁶⁷ However, no conflict involving the use of force is ‘outside the ambit’ of the prohibition of the use of force, regardless of jus in bello. The UN Security Council may use its powers under Chapter VII of the UN Charter to override certain aspects of international humanitarian law,Footnote ⁶⁸ but that is a separate question.

Churchill and Lowe present two alternative views on the use of force at sea.Footnote ⁶⁹ One ‘is that force may be lawfully used only with the authorization of the United Nations Security Council or alternatively in the exercise of the inherent right of self defence preserved by Article 51’, while the other is that ‘when force is used on a large scale it is regulated by the Laws of War and the law of neutrality’.Footnote ⁷⁰ However, these views do not have to be alternatives. The application of international humanitarian law and the prohibition of the use of force are not mutually exclusive on land, nor should they be at sea.

von Heinegg makes two apparently contradictory statements in one article. He states that ‘the illegality or legality under jus ad bellum has no impact on the illegality or legality under the jus in bello’,Footnote ⁷¹ which must be correct. However, he also writes that ‘efforts to limit the in bello legality in the light of the jus ad bellum have been futile’ and that if ‘a blockade is in compliance with the rules and principles of the law of air or naval warfare, its legality may not be doubted’.Footnote ⁷² These statements do not comport with the view presented in this article. von Heinegg refers to the ‘general consensus of states’, but Section 4.3 below shows that state practice instead favours the view that the use of force against ships may violate the prohibition of the use of force regardless of its compliance with international humanitarian law.

On the other hand, other writers discuss whether or how the use of force on the high seas can be legal as a form of self-defence.Footnote ⁷³ This constitutes a recognition that the two sets of rules apply concurrently, otherwise it would only have been necessary to discuss international humanitarian law.Footnote ⁷⁴ Moreover, some writers have claimed that specific threats or uses of force have been illegal precisely because they have been targeted against neutral ships outside territorial waters. Some writers have claimed this regarding the United Kingdom’s 1982 exclusion zone in the Falkland Islands,Footnote ⁷⁵ and others regarding Israel’s 2010 interception of neutral ships as part of its blockade of Gaza. ^{Footnote 76} Both of these situations are examined further in Section 4.3 below.

The legality of blockades has been discussed by the Prosecutor of the International Criminal Court. This came up in a report on the Situation on Registered Vessels of Comoros, Greece and Cambodia, which grew out of Israel’s aforementioned Gaza blockade enforcement. The Prosecutor’s conclusion was that although there was ‘a reasonable basis to believe that war crimes under the Court’s jurisdiction have been committed’, she would not bring the case before the Court, because ‘the situation would not be of sufficient gravity to justify further action’.Footnote ⁷⁷ When establishing whether war crimes had been committed, the Prosecutor claimed that a ‘blockading power may intercept and capture neutral vessels believed on reasonable grounds to be breaching the blockade’, with reference to the San Remo Manual.Footnote ⁷⁸ War crimes are defined by Article 8(2) of the ICC’s Rome Statute as ‘grave’ or ‘serious’ violations of international humanitarian law. Thus, the Prosecutor used the San Remo Manual to establish the content of international humanitarian law. The prohibition of the use of force was not involved, as it could have been if the prosecutor had discussed the crime of aggression. Therefore, the Prosecutor’s report cannot give any guidance on the relationship between the prohibition of the use of force and international humanitarian law.

In conclusion, the prohibition of the use of force applies to ships alongside international humanitarian law. states must comply with both sets of rules for a military action to be legal.

3.3 Self-defence against third states

As the prohibition of the use of force applies to ships alongside international humanitarian law, the use of force against neutral ships outside territorial waters must be justified by one of the three exceptions to the prohibition.Footnote ⁷⁹ The exceptions are self-defence, authorization from the UN Security Council, and consent from a host state.

The UN Security Council can authorize the use of force at sea as part of its powers under Chapter VII of the UN Charter.Footnote ⁸⁰ The states enforcing a Security Council resolution can target ships whose flag states are members of the UN. The Security Council cannot authorize the use of force against states that are not UN members,Footnote ⁸¹ but that has little practical relevance since all but a very few states have joined the UN.

The use of force against a ship can also be legal through consent from the flag state of a given ship.Footnote ⁸² However, as mentioned in Sections 2.2 and 2.4, the point of a blockade or exclusion zone is usually to stop all ships, so it is not practically possible for these measures to depend on consent. Visit and search operations can, by contrast, rely on consent from individual flag states.

What remains is the use of force in self-defence. Article 51 of the UN Charter recognizes a right to self-defence in the event of an ‘armed attack’. This is a higher threshold than ‘use of force’ in Article 2(4).Footnote ⁸³ The state that is using force on the high seas must have been the target of an ‘armed attack’ that can justify a defensive action. If so, the defensive action may involve the use of force at sea as well as on land.Footnote ⁸⁴

The ICJ has repeatedly stated that self-defence must be ‘necessary’ and ‘proportional’.Footnote ⁸⁵ The ‘necessary’ requirement means that the measures must be ‘necessary to respond to’ the armed attack.Footnote ⁸⁶ In some situations it could be ‘necessary’ to block naval access to an enemy territory as part of stopping an ongoing attack or to prevent a future follow-up attack, or it may be necessary to intercept and search specific ships. The proportionality requirement means that ‘self-defence would warrant only measures which are proportional’ to the armed attack.Footnote ⁸⁷ This must be assessed in each individual case. von Heinegg believes that ‘[i]ndiscriminate [maritime interdiction operations] exercised in vast sea areas would be disproportionate’.Footnote ⁸⁸ The conclusion in each case should depend on how ‘vast’ the area in question is and as well on other relevant circumstances. The graver the armed attack, the more extensive the response may be.

A civilian ship could engage in acts connected to a use of force or an ‘armed attack’, such as disrupting or destroying submarine cables or pipelines, wilfully causing large scale highly toxic pollution, landing saboteurs, deliberately ramming coastal state vessels, or supplying or supporting other vessels engaged in a use of force. The threshold for when such actions give a right to self-defence may be relatively low.Footnote ⁸⁹ The ICJ in Oil Platforms did ‘not exclude the possibility that the mining of a single military vessel’ could give a right to self-defence.Footnote ⁹⁰

Flag states generally have a right to use force to protect their own merchant ships ‘from unlawful attacks’ outside territorial waters.Footnote ⁹¹ This may be legal even if the merchant ships are subject to force below the threshold of an ‘armed attack’. In Nicaragua the ICJ entertained the possibility that a state may ‘use of force in reaction to measures which do not constitute an armed attack but may nevertheless involve a use of force’, but deliberately left the question unanswered.Footnote ⁹² In its earlier Corfu Channel decision, the Court rejected the United Kingdom’s argument that its mine sweeping operation could be classified as ‘self-protection or self-help’.Footnote ⁹³ However, as noted in Section 3.1 it is not clear whether the prohibition of the use of force was part of the Court’s reasoning. Judge Simma took a clear stance in his separate opinion in Oil Platforms, finding a right to take ‘proportionate defensive measures’ against ‘hostile action … below the level of Article 51’.Footnote ⁹⁴ Ruys concludes that the ICJ’s practice leaves it unclear whether ‘forcible counter-measures against less grave uses of force’ are permitted.Footnote ⁹⁵ A possible legal basis for a right to retaliate against attacks that do not constitute ‘armed attacks’ is unit self-defence.Footnote ⁹⁶ This is a right for individual military units to ‘use force under international law to defend themselves against attacks or threatened attacks’, as opposed to states doing so.Footnote ⁹⁷ This may be ‘an independent right recognized in customary international law’,Footnote ⁹⁸ with a different, usually lower, threshold in terms of gravity than UN Charter Article 51.Footnote ⁹⁹ Warships have the same right to unit self-defence as other military units.Footnote ¹⁰⁰ Unit self-defence may become increasingly relevant with the rise of ‘grey zone operations’ at sea, where it is difficult to say for sure whether an armed conflict exists or who is involved.Footnote ¹⁰¹

In short, if a neutral merchant ship were to engage in hostile acts against a warship, the warship would be allowed to defend itself by forcible measures, even outside territorial waters. However, the typical blockade or indiscriminate visit and search operation will mostly, if not exclusively cover ships that are not engaging in any hostile acts against the state or the ship conducting the operation. The state will instead be responding to an armed attack by another state. For the purposes of self-defence, the flag state of a neutral ship will be a third state.

The core of the right to self-defence is a right to use force against an attacking state. This should encompass the attacking state’s territory, including territorial waters, and armed forces, and others who participate in or contribute to the armed attack. Actions taken in self-defence may also affect third states.Footnote ¹⁰² The International Law Commission (ILC) recognized this when its commentaries to the Responsibility of States for Internationally Wrongful Acts Article 21 stated that ‘there may be effects vis-à-vis third States in certain circumstances’.Footnote ¹⁰³ Article 51 of the UN Charter does not say that measures taken in self-defence can only affect the attacking state.Footnote ¹⁰⁴

The commentaries to an earlier 1996 draft of the ILC’s text spoke of ‘indirect injury that might be suffered by a third State in connection with a measure of self-defence’.Footnote ¹⁰⁵ This was a comment to Article 34, which obliges states to pay full reparation for injuries caused by internationally wrongful acts. The comment pointed out that this did ‘preclude any wrongfulness’ of injury to third states. Thouvenin uses a similar terminology when speaking about ‘collateral breaches’ towards third states.Footnote ¹⁰⁶

In his Second Report on State Responsibility, Special Rapporteur James Crawford noted that states acting in self-defence have ‘certain belligerent rights, even as against neutrals’.Footnote ¹⁰⁷ The ILC’s work was not intended to ‘enter into’ the controversy that is at the heart of this article,Footnote ¹⁰⁸ i.e., whether ‘the traditional law of neutrality has survived unchanged in the Charter period’.Footnote ¹⁰⁹ Thus the commentaries to the final version of Article 21 explains that this question was meant to be left ‘open’.Footnote ¹¹⁰ Ultimately, the question of what a state acting in self-defence can do towards third states must be resolved without a clear answer from the ILC.

It may in practice be difficult to completely avoid ‘indirect injury’ or ‘collateral breaches’ towards neutrals as part of a self-defence measure directed at an attacking state. However, this is different from measures that are directed towards neutrals, such as blockades and indiscriminate visit and search operations. It is far less clear that the latter is permitted under the UN Charter.

Some guidance can be found in an analogy to the debate over the use of force against non-state actors on foreign soil. This debate centres on whether there is a right to self-defence against non-state actors operating on the territory of third states who are ‘unable or unwilling’ to prevent an ‘armed attack’ from the non-state actors.Footnote ¹¹¹ There is no consensus among states that such a right exists. A strong argument against such a right is that it would violate the territorial integrity of a third state. By contrast, the individual ships subject to blockades or indiscriminate visit and search operations are not committing or contributing to an armed attack, nor are their flag states facilitating it. Thus, there should be even less reason to recognize a right to use force against them in neutral waters than to attack non-state actors on foreign soil.

As a consequence, the logic and structure of the prohibition of the use of force suggest that force can only be used in self-defence against neutral ships that somehow engage in hostile acts against another state or its ships. If so, other ships can only be affected indirectly through collateral damage. A blockade or exclusion zone will by its nature cover every ship that the enforcing state is able to detect. That a ship does not voluntarily submit to inspection is not sufficient to believe it probable that the ship is engaging in hostile acts. Visit and search operations may, in contrast to blockades and exclusion zones, be limited to specific ships that are suspected of engaging in hostile acts.

However, broader rights to use force in self-defence against neutral ships outside territorial waters may be accepted if they are supported by state practice and opinio juris.Footnote ¹¹² Article 51 of the UN Charter is to be interpreted in line with the principles codified in the Vienna Convention on the Law of Treaties Article 31(3)(b), which says ‘subsequent practice in the application of the treaty which establishes the agreement of the parties regarding its interpretation’ is to be ‘taken into account’ when interpreting treaties.Footnote ¹¹³ The prohibition of the use of force is also customary international law and jus cogens, as noted in Section 1. That it is jus cogens means that it is less liable to change than regular rules of customary international law.Footnote ¹¹⁴ An argument may be made that using force against neutral ships is not a case of change in the law, since the law of blockades existed before the UN Charter. Interference with neutral ships was sometimes justified on the basis of self-defence even before the adoption of the UN Charter.Footnote ¹¹⁵ The next sections examine relevant state practice, where states have attempted to use of force against neutral ships outside territorial waters. The overall picture is that states have not accepted the indiscriminate use of force against neutral ships outside territorial waters in the post-UN Charter world.

4.1 Self-defence is sometimes not invoked

The following section examines state practice and opinio juris regarding the use of force against neutral ships outside territorial waters. This article focuses on the relationship between international humanitarian law and the prohibition of the use of force. The prohibition of the use of force was established in 1945, through the UN Charter. The following sections are therefore limited to post-1945 practice. Earlier practice is not relevant to the argument in this article.Footnote ¹¹⁶

Some threats or uses of force have been justified by reference to other legal rules than the right to self-defence in UN Charter Article 51.

The Security Council has authorized naval blockades, for example against Iraq (1990),Footnote ¹¹⁷ Yugoslavia (1992),Footnote ¹¹⁸ and Libya (2011).Footnote ¹¹⁹ These were enforced outside territorial waters. Their legal basis was UN Charter Article 42, not Article 51. This shows that if states believe it necessary to be able to use force against (ships from) any state, they have the option of going through the Security Council instead of trying to rely on the right to self-defence.

During the Cuban Missile Crisis of 1962, the United States established a blockade around Cuba with the aim of preventing the Soviet Union from stationing nuclear missiles on the island.Footnote ¹²⁰ An official declaration by President Kennedy stated that ships that ‘may be proceeding towards Cuba’ would be intercepted, presumably in any maritime zone.Footnote ¹²¹ A reasonable interpretation of the President’s declaration is that it constituted a standing threat of the use of force against neutral ships outside territorial waters.Footnote ¹²² Eventually the US intercepted only a single third-state ship, flying a Lebanese flag, and it is not clear exactly where this occurred.Footnote ¹²³ For the purposes of this article it is notable that the US did not invoke a right to self-defence against the Soviet Union or Cuba.Footnote ¹²⁴ The use of force was instead justified with reference to UN Charter Article 53, which mentions ‘regional arrangements or agencies for enforcement action’, and Article 6 of the Rio Treaty,Footnote ¹²⁵ covering ‘an aggression which is not an armed attack’.Footnote ¹²⁶ This legal strategy has not been accepted or repeated by later international lawyers. At the time third states generally did not comment on the legality of the declaration.Footnote ¹²⁷ The US could not legally use of force in self-defence, since there was no armed attack.Footnote ¹²⁸ In any case, since the US did not invoke self-defence, the incident has little if any precedential value for the argument in this article. Some US scholars have claimed that the Cuban Missile Crisis had ‘the greatest influence on the further development of doctrine concerning maritime interdiction’,Footnote ¹²⁹ but that does not apply to the points taken up in this article.

Maritime interdiction has also been used in post-2001 antiterrorism campaigns such as Operation Active Endeavour and Operation Enduring Freedom. The US-led invasion of Afghanistan was justified as self-defence against the terrorist attacks of 11 September 2001.Footnote ¹³⁰ However, many of the naval operations were instead based on ‘flag State consent’.Footnote ¹³¹ NATO operations against FR Yugoslavia during the Kosovo War (1999) involved a ‘voluntary visit and search regime’.Footnote ¹³²

4.2 Acceptance of operations limited to territorial waters

In practice the use of force against neutral ships is often limited to territorial waters.Footnote ¹³³ This means that there is little state practice that can support a right to use force in self-defence against neutral ships outside territorial waters. Moreover, while states may protest the legality of operations within territorial waters, they generally do not claim that such operations by themselves violate the prohibition of the use of force. This is in contrast with how states react to the use of force outside territorial waters, as shown in the sections below.

The relative frequency of operations limited to territorial waters also shows that naval operations may be militarily effective even though the enforcing state does not use force outside territorial waters. This weakens the potential argument that the use of force outside territorial waters should be permitted because it will often be necessary in order to achieve a military aim.

As part of the Korean War (1950–1953), the UN established a blockade of North Korea in order to ‘deny unauthorized ingress and egress from the Korean coast [and] suppress seaborne traffic to and from North Korea and to prevent movement by sea of forces and supplies for use in operations against South Korea’.Footnote ¹³⁴ The operation was ordered by President Truman of the United States and authorized by the UN Security Council.Footnote ¹³⁵ The legal basis for the UN operations against North Korea were either collective self-defence or authorization by the UN Security Council.Footnote ¹³⁶ Drew reports that ‘in 1950 UN vessels sank some 213 junks and sampans and damaged 147 more, while capturing nine’.Footnote ¹³⁷ However, the operations were enforced around Korean ports, not outside territorial waters.Footnote ¹³⁸

India blockaded Bangladesh’s (then known as East Pakistan) coast in the Bangladesh War (1971).Footnote ¹³⁹ von Heinegg reports that ‘six merchant ships and numerous small boats were captured’ and that ‘[v]essels that did not comply with the orders by the warships’ commanders were attacked and sunk.Footnote ¹⁴⁰ India invoked self-defence under UN Charter Article 51.Footnote ¹⁴¹ The blockade was ‘generally not enforced on high seas’,Footnote ¹⁴² but there was ‘one Liberian ship sunk 26.5 miles off coast’.Footnote ¹⁴³ The legality of the latter operation was not publicly discussed by the parties.Footnote ¹⁴⁴

As part of the Vietnam War (1955–1975), the United States mined the city of Hai Phong in 1972. A speech by US president Nixon explained that ‘[a]ll entrances to North Vietnamese ports will be mined’ and threatened ‘appropriate measures within the internal and claimed territorial waters of North Vietnam’.Footnote ¹⁴⁵ In the end ‘[n]o foreign merchant vessels were sunk by the minefields’,Footnote ¹⁴⁶ and the operation ‘did not involve action on the high seas’.Footnote ¹⁴⁷ The legality of many aspects of the United States’ actions in Vietnam were controversial,Footnote ¹⁴⁸ but the potential harm to neutral ships does not seem to have been singled out for criticism.

The 2006 Lebanon War included a blockade of Lebanon by the Israel.Footnote ¹⁴⁹ A report by the UN Human Rights Council noted that this was ‘a comprehensive blockade of Lebanese ports and harbours’.Footnote ¹⁵⁰ Israel’s official justification was that ‘[t]he ports and harbours of Lebanon are used to transfer terrorists and weapons’, which means that the official justification was limited to ports and harbours and did not apply to areas outside territorial waters.Footnote ¹⁵¹ Israel invoked self-defence under UN Charter Article 51,Footnote ¹⁵² but other states were divided on whether this was correct.Footnote ¹⁵³ The states did not specifically comment on the effects of neutral shipping.

4.3 Protests against operations outside territorial waters

Section 4.2 above showed that various states have used force against neutral ships inside territorial waters, and that other states have not specifically protested against this. This section will show that some states have used force against neutral ships outside territorial waters, but that this often has been met by protests from other states.Footnote ¹⁵⁴

During the Algerian War (1954–1962), France searched neutral ships in order ‘to stem the flow of arms and munitions into Algeria’.Footnote ¹⁵⁵ In all, ‘thousands of ships’ were stopped and searched.Footnote ¹⁵⁶ The operations mainly took place within a zone that extended ‘twenty to fifty kilometres from the coast of Algeria’,Footnote ¹⁵⁷ well beyond the territorial waters of French Algeria, and searches were conducted as far away as the English Channel.Footnote ¹⁵⁸ O’Connell reports that ‘[t]he ships of thirteen European countries were interfered with on the high seas’.Footnote ¹⁵⁹ France’s actions were ‘vigorously opposed by many of the States whose ships were affected’.Footnote ¹⁶⁰ France ‘became involved in more or less serious diplomatic difficulties’,Footnote ¹⁶¹ as well as legal actions in French courts.Footnote ¹⁶² The protests focused on the ‘unlawful interference with the freedom of navigation’ as well as the extensive operations being ‘disproportionate to the threat posed’.Footnote ¹⁶³

The United Kingdom established a 200 nautical mile ‘total exclusion zone’ around the Falkland Islands during the Falklands War (1982). Within this zone any ship, including those of third states, would be liable to attack if their presence was not authorized by the UK government.Footnote ¹⁶⁴ The UK measure was prompted by Argentina’s 1982 invasion of the Falkland Islands, which violated UN Charter 2(4) and gave the UK a right to self-defence under Article 51.Footnote ¹⁶⁵ states’ reactions to the UK’s measure were split along familiar Cold War lines. Argentina, its Latin American allies, and communist states condemned it, while the UK’s NATO allies voiced support.Footnote ¹⁶⁶ There is little doubt that enforcing the exclusion zone towards neutral ships by targeting them would have violated international humanitarian law.Footnote ¹⁶⁷ For the purposes of this article it is notable that some states objected to the mere threat of targeting neutral ships outside territorial waters.Footnote ¹⁶⁸

During the Iran-Iraq War (1980–1988), both parties established what were in effect exclusion zones.Footnote ¹⁶⁹ Iran’s zone ran along the Iranian coast along the length of the Persian Gulf,Footnote ¹⁷⁰ but Iran searched and visited a significant number of neutral ships outside territorial waters.Footnote ¹⁷¹ From 1982, Iraq’s zone extended up to 65 km from Kharg Island, and from 1986 it reached ‘close to Kuwaiti territorial waters’.Footnote ¹⁷² Iraq’s zones thus reached well beyond the warring states’ territorial waters. As the conflict dragged on, the two states’ visit and search operations morphed into large-scale unprovoked attacks on neutral ships outside territorial waters.Footnote ¹⁷³ Both states invoked self-defence under UN Charter Article 51.Footnote ¹⁷⁴

The outright attacks on neutral ships were rightly condemned by other states and by the UN Security Council.Footnote ¹⁷⁵ Reactions to the visit and search and operations were more nuanced. Some states ‘acknowledged the belligerent right of visit and search’,Footnote ¹⁷⁶ while others opposed it.Footnote ¹⁷⁷ The UK position is particularly interesting. The UK ‘refus[ed] to accept the idea that “belligerent rights” continued to exist after the adoption of the UN Charter’.Footnote ¹⁷⁸ The UK official position referred to ‘Article 51’ and acknowledged that a state is:

The official position adds that the ‘right would not extend to the imposition of a maritime blockade or other forms of economic warfare’.Footnote ¹⁸⁰ The UK’s position is in line with the argument presented in this article, in that rights under international humanitarian law must be exercised in compliance with the UN Charter, and that indiscriminate visit and search operations or blockades against neutral ships outside territorial waters cannot be reconciled with the prohibition of the use of force.

Israel has imposed a naval blockade on Gaza since 2007. A particularly contentious consequence of the blockade was the Israeli Navy’s forcible boarding of a ‘Gaza flotilla’ in May 2010. The ships flew the flags of various third states: The Comoros, USA, Turkey, Greece, and Kiribati. Israel’s actions took place 72 nautical miles off the coast of Israel, which is in Israel’s exclusive economic zone and well outside its territorial waters. Whether Israel has a right to self-defence against Gaza or Palestine at all is debated.Footnote ¹⁸¹ It is also debated whether Israel’s blockade of Gaza generally or the flotilla raid specifically complied with international humanitarian law.Footnote ¹⁸²

Israel’s actions were widely criticized by other states and international organizations.Footnote ¹⁸³ Many of the statements point specifically to use of force against neutral ships on the high seas as a problem. For example Turkey’s foreign minister held that ‘freedom of navigation, was one of the oldest forms of international law; no vessel could be stopped or boarded without the consent of the captain or flag State’.Footnote ¹⁸⁴ The President of the European Parliament stated that the ‘interception of the convoy in international waters’ was ‘a clear and unacceptable breach of international law’.Footnote ¹⁸⁵ The event was discussed in the UN Security Council, where Mexico’s representative on the UN Security Council stated ‘condemned in the strongest terms the armed attack by Israeli forces in international waters against the civilian flotilla’.Footnote ¹⁸⁶ The representatives of Brazil, Austria, Lebanon, and Palestine also specifically highlighted that the attack took place in international waters,Footnote ¹⁸⁷ as did a statement by the UN Special Coordinator for the Middle East Peace Process and Commissioner-General of the UN Relief and Works Agency.Footnote ¹⁸⁸ The intense criticism of Israel’s action and the repeated emphasis on the target being a neutral ship outside territorial waters supports the view that Israel’s actions violated the prohibition of the use of force even though international humanitarian law recognizes a right to blockade.

The incident was the subject of several official reports. The UN Secretary General appointed a panel that investigated the incident and produced the so-called Palmer Report. The panel’s terms of reference did not cover the legal aspects of the incident,Footnote ¹⁸⁹ but an annex dealing with legal questions stated that using force against a foreign flagged ship is legal if ‘used in self-defence, in line with Articles 2(4) and 51 of the U.N. Charter’. ^{Footnote 190} That is correct, but the report does not delve deeper into what connection there was between the neutral ships and a possible ‘armed attack’ against Israel.

A commission appointed by the UN Human Rights Council produced the ‘Goldstone Report’, but it did not discuss with the legality of using force against foreign-flagged ships.Footnote ¹⁹¹

The Human Rights Council produced another report that dealt with the legality of Israel’s actions. The Report invoked ‘the San Remo Manual and a number of military manuals’ and concludes that ‘a right to visit, inspect and control the destinations of neutral vessels on the high seas’ exists only ‘upon reasonable suspicion that a vessel is engaged in activities which support the enemy’.Footnote ¹⁹² However, the Report also stated that ‘if there is no lawful blockade’, intercepting a vessel is legal if it ‘was making an effective contribution to the opposing forces’ or if it is done in self self-defence.Footnote ¹⁹³ This seems to build on an assumption that the prohibition of the use of force and humanitarian law are alternative legal regimes at sea, and that they do not apply concurrently to neutral ships. However, as explained in Section 3.2, the use of force against neutral ships must be justified under the prohibition of the use of force, regardless of whether they comply with international humanitarian law.

An official Turkish report on the same incident stated that Israel’s actions amounted to ‘unlawful use of force’.Footnote ¹⁹⁴ Israel’s official Turkel Commission Report did not discuss the use of force under the UN Charter.Footnote ¹⁹⁵

In the aftermath of the operation and the resulting criticism, Israel seemed to change its approach. In July 2010, Israel held off intercepting a Libyan ship until it reached Gaza’s territorial waters.Footnote ¹⁹⁶ If Israel has a right to use self-defence against Gaza, it has a right to use of force on the territory and in the territorial waters of Gaza. Israel’s altered practice is in line with the argument presented in this article.