How International Law Restricts the Use of Military Force in Hormuz

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We await whether an allied action will protect shipping in the Persian Gulf, and whether it will be led by the USA or by European states. The UK’s new government will support US action, while at least some other European states are reluctant to be seen as supporters of US aggressive policy towards Iran. Political arguments aside, there are important international law concerns with participation in such action, whether American or European-led (see also this recent post by Hartwig).

Absence of a Security Council mandate

The first concern is that such an action would not have a UN mandate. The Security Council can authorize military actions to ensure peace and security, even setting aside other rules of international law. Admittedly, protection of shipping might not fall under the Security Council’s competencies to maintain peace and security. Regardless, a mandate for a military action in the Persian Gulf is in any case politically unlikely.

The law of the sea

Without a mandate from the Security Council, there are strong arguments against the legality of such action. The Strait of Hormuz is not international waters. Hormuz is an international strait covered by the territorial waters of Iran and Oman (see Gioia). The coastal states have sovereignty in the territorial sea. Other states have an extensive right to transit, known as transit passage, in such international straits according to the UN Convention on the Law of the Sea (UNCLOS). Iran, though, has not acceded to UNCLOS, claiming that other states’ vessels have less extensive rights of navigation, namely the right of innocent passage. Let us stick to transit passage for the sake of simplicity.

The sovereignty of coastal states in the territorial sea means that they can establish laws and enforce them, absent special restrictions in international law. However, coastal states cannot prevent passage through international straits. This also applies to the passage of military vessels. There are also limits to the legislation and enforcement the coastal state can apply to navigation. But it is established that the right of transit passage does not change the legal status of the areas. So they continue to be territorial waters.

Duties for the passage are also established. The purpose shall exclusively be continuous and expeditious transit, and the vessels shall refrain from any other activity. Ships shall not threaten or use force against the strait states. This means that while military vessels have the right to sail through the Strait of Hormuz, they cannot use military force during the passage – not even to protect their own ships.

Use of force

One option available to states affected by illegal measures by Iran is to implement lawful countermeasures, such as sanctions. However, the only possible basis for a response involving a use of force not authorized by the Security Council, is an exercise of self-defence coordinated by the ship’s flag state. The use of self-defence as a measure against attacks on commercial vessels was discussed in the International Court of Justice’s (ICJ) Oil Platforms case.

This case concerned the ‘tanker war’, connected to the war between Iraq and Iran 1980-1988. The two states attacked oil tankers in the Persian Gulf, including vessels flying the US flag. US naval vessels were also damaged. This led to US bombing and destruction of Iranian oil platforms. Iran instituted proceedings against the USA in 1992 and the final judgment was issued in 2003.

The USA never denied that its actions against the Iranian platforms amounted to use of force under the UN Charter art. 2(4) (para. 45). However, the Court upheld the Nicaragua distinction between the use of force and an ‘armed attack’ under the Charter art. 51. Armed attack only includes ‘the most grave forms of the use of force’ (para. 51).

The Court discussed whether the attacks could be attributed to Iran and whether they were aimed specially at US vessels. However, it also addressed – setting aside the issue of attribution – whether the attack on the tanker Sea Isle City, either in itself or in combination with other attacks cited by the USA, could be categorized as an armed attack, justifying self-defence.

The Court first noted that the missile hitting Sea Isle City in Kuwaiti waters, if fired from Iran, could not have been aimed at this specific vessel. Another tanker, the Texaco Caribbean, was not flying US flag, so an attack on this vessel could not be seen as an attack against the USA. No persuasive evidence had been produced that firing on US helicopters came from Iran gunboats or oil platforms. There was no evidence that mines were aimed specifically at the USA or that the mine struck by the vessel Bridgeton was laid to harm that ship or other US vessels. Ultimately, the Court concluded:

Even taken cumulatively, and reserving, as already noted, the question of Iranian responsibility, these incidents do not seem to the Court to constitute an armed attack on the United States, of the kind that the Court, in the [Nicaragua case], qualified as a “most grave” form of the use of force (para. 64).

Hence, the ICJ did not explicitly exclude the possibility that attacks on commercial vessels could be considered armed attacks. The fact that the Court considered several factors related to the attacks, could also be seen as supporting an argument that attack on such vessels might be an armed attack.

On the other hand, the Court never stated explicitly that such attacks could qualify. Instead, the Court discussed the attacks in terms of whether they qualified as the ‘most grave’ forms of the use of force. This contrasts to the mining of the military vessel Samuel B. Roberts. Here, the court did ‘not exclude the possibility that the mining of a single military vessel might be sufficient to bring into play the “inherent right of self-defence”’ (para. 70). The implication seems to be that attacks on a single commercial vessel could never qualify as an armed attack. At the most, attacks on commercial vessels would only qualify if they, in the overall assessment of the situation, could be considered as the ‘most grave’ forms of use of force. This would conform to a distinction based on the interests attacked. Offensives against military vessels are directed at the state itself. In contrast, attacks on commercial vessels should not be seen as aimed against the state as such, unless they are of a magnitude that would threaten the state’s security interests.

Scholarship discussing Oil Platforms is equivocal. Momtaz (Professor at Tehran University and Counsel for Iran) believes that the Court found that the attack on Sea Isle City did not qualify as an armed attack. On the other hand, Raab (then legal adviser to the British Embassy, the Hague and now British Foreign Minister) argues that the Court ‘implied – but did not explicitly state – that an attack on merchant shipping could, in principle, be regarded as an armed attack against the flag state’. He emphasizes that while Nicaragua was concerned with the UNGA Definition of Aggression art. 3 (g) on sending of cross-border irregular forces, the Definition art. 3 (d) also sets out what constitutes maritime aggression: ‘An attack by the armed forces of a State on the land, sea or air forces, or marine and air fleets of another State’. He asserts that this definition suggests that ‘any attack on a merchant ship constitutes an act of aggression against the flag state. In any event, any attack on a ship could be interpreted as amounting to an attack on the fleet, if it is intended to treat all the ships comprising the fleet as one integral unit’. Ochoa-Ruiz and Salamanca-Aguado conclude that the Court has not clarified ‘whether actions against commercial vessels and aircraft can be equated with attacks on the state itself’.

Subsequent writings on the use of force confirm the ambiguous legal situation. Kress finds that the Oil Platforms ‘did not seem to exclude the possibility of an armed attack being directed against a merchant vessel of a state’. Gray concludes that there is ‘considerable doubt as to whether a single attack on a merchant vessel (as opposed to a military vessel) could constitute an armed attack on a state and the Court itself did not directly address this issue’. Finally, Nolte and Randelzhofer find that the case seems to give a certain support to holding that attacks against a single commercial vessel could trigger the right of self-defence. However, the safer interpretation is that the Court evaded the issue and dealt with the allegations on evidentiary grounds. Like Raab, they refer to the Definition of Aggression art. 3 (d), albeit with a somewhat different interpretation:

‘as distinct from individual commercial vessels or aircraft, assaults on the whole of the civilian marine or air fleet, as purportedly referred to in Art. 3 (d) of the Definition of Aggression, are said to threaten the affected State as such and thus, if applicable, to constitute “armed attacks”. The same should be true for continuous assaults on essential parts of them’ (emphases added).


Which conclusions can we draw about the legal possibilities to protect shipping through Hormuz?

First, there is no prohibition in the law of the sea against military vessels escorting merchant ships. Illegal actions by Iran against shipping can also be protected by the use of countermeasures not involving the use of force. But caution is advised in use of military force in Hormuz. This would at the outset violate the prohibition against use of force in the UN Charter art. 2(4). The only plausible legal ground for use of force in this situation would be the right to self-defence against armed attack.

However, Iranian arrest of ships – even if unlawful – cannot be seen as an armed attack (see Guilfoyle). Whether military action against commercial vessels could ever be seen as an armed attack is an unresolved issue. But if so, the best reasons militate against seeing an attack against a single ship as an armed attack. The ICJ’s decision in the Oil Platforms case suggests that attacks on commercial vessels only qualify as armed attacks if they are of such gravity that they threaten the state’s security interests. This is supported by the Definition of Aggression art. 3 (d) on attacks on the marine fleet of another state. As we have seen, a reasonable interpretation is that this provision requires attacks on the fleet as a whole or continuous attacks on essential parts of it.


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Ayman Salama says

August 29, 2019

Dear Geir
I congratulate you on addressing this very complicated issue that embraces both International Law of the Sea and Public International law , but you paid thoughtful concern to the latter on the cost of the International Law of the Sea though it is the cornerstone of the whole confrontation in the strait of Hormuz .Legal deliberations and interventions in relation to the overlapping and correlations of the strait states ( Iran and Oman )rights under International law and their domestic legislation and statues and mostly all foreign states under international law are still are still insufficient to scholars and researchers .
Therefore , I will address the most substantial and essential landmarks in this context

1- Conventional international law had been established on customary international law and most provisions of UNCLOS are basically derived from international maritime customs
2- UNCLOS re-affirmed coastal state sovereignty in the territorial sea and besides the strait state in other provisions of the convention ( This had become uncontroversial customary international norm )
3- Sultanate of Oman- a state party to the convention - made a declaration when she signed the convention that foreign military vessels shall obtain a clearance prior to transit passage through strait of Hormuz and Iran that has not ratified the UNCLOS has the same legal stance .
4- Innocent passage of foreign aircraft and ships that seek to exercise the right of innocent passage are not entitled , among other activities prejudicial to the security and order of the strait states
5- Widening the territorial seas of coastal and strait states to 12 nautical miles benefited the utmost of Iran and Oman interests , thus prior to the convention endorsed in 1982 Iran would have not claimed her customary international rights especially that are consistent with the convention
6- USA , the non-stat party to the convention can not make use of certain articles from the convention to achieve her international dominance to reshape international law of the sea and attain her own different national interests
7- Iranian official statement annexed to her signature of the convention that : States that can exercise transit passage in Hormuz Strait are those that are parties to the UNCLOS must not completely disregarded because such a prerogative must not only base upon customary international law
8- Iran the non state party to the convention must not exploit certain provisions of the convention such articles : 41 and 42 to attain her own interests and , adversely and at the same time does not comply by other articles of the convention like article 44.
9- In the vein of the two rivals : Iran and USA , both can not claim interpretation of UNLCOS but claim only to customary international law , UN Charter and other international instruments regarding law of the sea and both of the two states are parties to these instruments .

Dr Ayman Salama

Geir Ulfstein says

August 30, 2019

Dear Ayman,
You are right that I mainly concentrated on general international law on the use of force, rather than on the law of the sea. I could have expanded on the possible difference of the right of passage based on customary international law and the law of the sea convention (UNCLOS), particularly since Iran (and the USA) has not ratified the convention - and the right of passage as discussed in the Corfu Channel case. It is also interesting to discuss what should be considered prohibited according to the requirement that ships - including military vessels - in international straits should 'refrain from any activities other than those incident to their normal modes of continuous and expeditious transit' (UNCLOS, art. 39(1)(c)).
Best, Geir

Kriangsak Kittichaisaree says

September 4, 2019

This interview with Judge Wolfrum is very interesting:

Charles Dunlap says

September 9, 2019

Interesting posts, except that it is completely untrue that a warship cannot act in self-defense to protect itself from attack during an otherwise lawful passage through an international strait.

Geir Ulfstein says

September 10, 2019

No arguments are presented by Charles Dunlap. Besides, my main point was whether attacks on commercial vessels - not warships - could be seen as armed attack and therefore provide a basis for self-defense under international law.