Expert blog: What legal right does Iran have to ‘close’ the Strait of Hormuz?

Since 28 February, Iran has come under attack from both the US and Israel, under the heading of “Operation Epic Fury”.

Iran’s response, so far, has focused on targeting US bases and interests in the Middle East, while also significantly disrupting the flow of shipping traffic through the Strait of Hormuz by attacking passing ships and reportedly laying mines, sending economic shockwaves through a global economy that relies on the transit of oil through the region - around 20% of the world’s oil and gas passes through the Strait.

But what are the legal ramifications of Iran’s response?  Does it have the right to, effectively, “close” the Strait, and to attack traffic passing through it?

What international law says

The short answer is that Iran does have a limited right to respond in self-defence, but only against military targets and within strict limits, and that any attempt to completely close the Strait of Hormuz would be difficult to justify.

The Strait of Hormuz connects the Persian Gulf to the Gulf of Oman and is bordered to the north by Iran and to the south by Oman and the UAE. The Strait is 21 nautical miles (around 39km) at its narrowest point and, given that both Iran and Oman claim a 12 nm territorial sea (as permitted under the1982 UN Convention on the Law of the Sea), ships will need to pass through Iranian and/or Omani territorial waters.

Under international law, however, there is no right for States to “close”, as such, their territorial waters where this would impede the use of “straits used for international navigation”.

The UN Convention on the Law of the Sea makes clear that “all ships and aircraft enjoy the right of transit passage, which shall not be impeded” and that “States bordering straits shall not hamper transit passage.”

Iran is not a party to the Convention, but the obligations here are considered to be part of customary international law - therefore binding nevertheless - and Iran has in any case indicated, in the past, that it recognises this position.

The key question, then, is whether the attacks against Iran shift this status quo.

Self‑defence, proportionality and the limits of legitimate force

The legality of the US/Israeli attacks is, at best, deeply questionable, with justifications for the use of force ranging from the elimination of future threats from Iran to arguments based around humanitarian intervention or regime change.

Article 2(4) of the United Nations Charter is clear in forbidding the use of armed force between States. While Article 51 of the Charter permits use of force in self-defence, there is nothing (publicly available) to suggest the imminence of any such military threat, certainly since the “Twelve Day War” between these powers in 2025. Similarly, there is no clearly established right to humanitarian intervention under international law.

While States have, sometimes, claimed a right to use force on humanitarian grounds, the suggestion that governments can unilaterally determine their right to use force in such circumstances, is inherently problematic. Similarly, there is no legal basis to the use of force to enact regime change. This is in no way to defend the Iranian regime’s treatment of its own population, which is undoubtedly abhorrent.

Against this backdrop, Iran has a limited right to defend itself from attack.  It may do so wherever necessary (i.e. where it has no choice but to use force to repel the threat) and in a manner that is proportionate (i.e. that goes no further than is actually needed to repel the threat). On this basis, strikes against US/Israeli military bases and infrastructure in the region would be, in principle, permissible.

However, attacks intended to target civilians or civilian areas/infrastructure would be war crimes, regardless of the overarching nature of Iran’s military engagement. Attacks intended as “retaliation”, such as those against Qatar’s Ras Laffan gas field, are likewise not permitted under international law.

The laws of armed conflict

Under the laws of armed conflict, in the context of the situation in the Strait of Hormuz, Iran would be permitted to attack, where necessary to defend itself: (a) US/Israeli military vessels; (b) merchant vessels if the cargo/oil is intended for military use by States attacking Iran (where they make an effective contribution to military action and their destruction offers Iran a definite military advantage); (c) any vessels “sailing under convoy of enemy warships” (in other words, any ships that the US or Israeli navies escort through the Strait could be considered as legitimate targets by Iran).

Vessels that are neutral, i.e., that have no role in the conflict, should not be attacked or impeded. It is apparent that several (if not a majority of) attacked vessels do fall into this category. Moreover, the deployment of naval mines in a major international strait would be problematic due to their inherently indiscriminate nature and the obligation to ensure safe navigation for neutral shipping.

When launching any attack, even against permitted targets, Iran must also aim to minimise civilian collateral damage and any environmental fallout, excessive levels of which would constitute the commission of a war crime.

It is apparent that Iran has not, in fact, enacted a complete closure of the Strait, offering safe passage to vessels flagged to China, India, and Turkey, for instance while insisting the closure only applies to the US, Israel, and their “allies”.

The question of whether Iran could indeed, in law, enact a complete and true “closure” of the Strait would hinge on the question of “necessity”, based on overall level of threat posed to Iran, what such a closure would seek to achieve, and whether such an action, with the global consequence that it entails, could be justifiable.

Dr Mark Chadwick, Principal Lecturer, Nottingham Law School