Passage Through Hormuz in a Time of War and Peace

On 28 February, the US and Israel bombed Iran. In response, Iran closed the Strait of Hormuz to the passage of all shipping. This has caused a crisis in energy supply chains, with serious economic consequences for countries, including Asian ones, which rely heavily on the strait for their energy supplies. To compound matters, US President Donald Trump announced a sweeping blockade of Iranian ports, which would impede the passage of all maritime traffic entering and exiting the strait. Neutral states are caught in the crosshairs of a US-Israel war against Iran with the Hormuz Strait as a chokepoint. This has literally choked the passage of shipping. Some perspective is needed here: under international law, in straits used for international navigation, there is a clear right of unimpeded passage for all foreign-flagged vessels. The devil, however, is in the details in times of war.

The Strait of Hormuz is bordered by Iran and Oman. It is among the most critical chokepoints in the world, with some 20 per cent of global oil supplies passing through it, much of it headed for Asia. This is not the first time the Strait of Hormuz has been at the centre of a crisis. Merchant shipping in the Hormuz Strait came under attack during the Iraq-Iran War (1980-1988) in what is known as the Tanker Wars. Tensions in the Strait of Hormuz continued to simmer after the war ended and intensified after the first Trump administration withdrew from the Joint Comprehensive Plan of Action in 2018. However, despite making threats, Iran has never closed the strait to the passage of all ships, that is, until the US and Israel attacked Iran on 28 February. Experts deem this attack a clear violation of the UN Charter.

Since then, the Strait of Hormuz has dominated the headlines and impacted the economies of countries around the world. Southeast Asia has not been spared, with several countries experiencing energy shortages, such as Sri Lanka and Thailand. The Philippines, which has declared a national emergency, has even sought to negotiate passage with Iran. This has sparked controversy among ASEAN states. Singapore has spoken out against negotiating with Iran over passage through the strait and vigorously defended the case for passage as a matter of a protected transit right under international law and principle.

What then is the legal status of the Hormuz Strait? It is often referred to as an “international strait”. This is not correct. It is a strait used for international navigation, but it is not “international”: it is part of the territorial waters of Iran and Oman and subject to their sovereignty. Nonetheless, this does not mean Oman or Iran can prevent ships from passing through the strait as international law provides specific rules for straits.

In general, a strait State (a coastal nation-state bordering a strait used for international navigation) must permit passage to all foreign-flagged vessels. However, there are some legal nuances. The principal international convention to regulate activities at sea is the 1982 United Nations Convention on the Law of the Sea (UNCLOS). The wrinkle here is that Iran has signed but never ratified it. Oman, however, is a party to it. This means Iran is not legally bound by the provisions of UNCLOS. That said, however, it cannot act against the objective and purpose of the Convention.

… if Iran can attack ships flying under the flag of the enemy in the strait it borders, and those aiding the enemy, it can prevent or at least threaten their passage — but not that of ships navigating under the flag of neutral States.

Before UNCLOS, passage through the territorial waters of a strait used for international law was non-suspendable innocent passage, and maritime passage in the high seas constituted freedom of navigation. To reach an agreement during the UNCLOS negotiations, states sought to extend their territorial sea to 12 nautical miles. As a result, part of the high seas became territorial seas. As a compromise for the major maritime powers, such as the US, which insisted on preserving their freedom of navigation rights, UNCLOS introduced a new regime for passage called “transit passage”. There are differences between the two regimes as it pertains to issues such as submarine passage and overflight. In both cases, however, a strait State cannot prevent passage or impose conditions on such passage. This is particularly pertinent as it pertains to the imposition of tolls, which is not permissible.

The other wrinkle here is that Iran does not accept transit passage as a rule of customary international law, as other states such as Singapore do. When Iran signed UNCLOS, it made a written declaration that transit passage only applies to State Parties to UNCLOS because it was the outcome of a negotiated “package deal”. Whether Iran is a persistent objector against transit passage under international law and is, by extension, exempt from it is another matter altogether. What is important is that under both transit passage and non-suspendable innocent passage, unless there is an exception, all ships have the right to navigate through the strait so long as such passage is peaceful.

This leads us to another wrinkle: when the US and Israel attacked Iran, the laws of armed conflict were triggered. Naval warfare, including strait passage, is covered by customary international law, which is found in the San Remo Manual on International Law Applicable to Armed Conflicts at Sea. The laws of armed conflict divide parties into two camps: belligerents and neutrals.

According to the San Remo Manual, the right of transit passage and non-suspendable innocent passage continue to apply even in times of armed conflict. However, there is one exception: if the strait State is engaged in an armed conflict, as is Iran with the US and Israel, it can attack vessels flying under the flag of the enemy, and those vessels that are assisting the enemy in the strait. This is subject to the rules of armed conflict — that is, distinction, proportionality, military necessity and humanity. Nonetheless, even certain enemy vessels are exempt from attack, such as hospital ships, vessels engaged in humanitarian missions and a limited number of other categories.

In practical terms, this means that if Iran can attack ships flying under the flag of the enemy in the strait it borders, and those aiding the enemy, it can prevent or at least threaten their passage — but not that of ships navigating under the flag of neutral States.

In the many years of war and political turmoil, Iran has never entirely closed the Strait of Hormuz to shipping, although it has made it difficult at times. Iran’s extreme measures are a direct consequence of the recent armed attack against it by the US and Israel. This is impacting states as far as the ASEAN region, creating economic havoc and new political divisions. Under international law, Iran is required to allow passage — whether innocent or transit — to all neutral shipping.

The announcement by the US to impose a naval blockade on Iranian ports has given the saga a new twist. Speaking to Channel News Asia, James Kraska, a US naval law expert, justified the US decision to blockade Iranian ports and explained it would apply to all shipping. Such an extreme measure, rather than opening the strait, would add another layer of disruption to unimpeded passage rights in the Strait of Hormuz. While in general, the rules of naval warfare allow for blockades of enemy ports, the devil is in the details. What is certain that such a blockade risks exacerbating an already stressed merchant shipping in the Persian Gulf.

Singapore has rightly defended international law, which provides for a stable world order. The failure by States to apply the law has created the chaos and instability the world is currently experiencing.

2026/114