The Hormuz crisis: are the Iran and US blockades illegal?

The Strait of Hormuz has become the centre of a global legal and geopolitical crisis. Two blockades, one Iranian, and one from the United States, now contest control of a waterway through which roughly a fifth of the world’s oil passes.

Both actions raise fundamental questions under international law. Both states involved have complicated relationships with the treaty that governs the seas. The answers matter not just for the ships caught in the crossfire, but for the entire architecture of international maritime order.

The law: what UNCLOS says

The United Nations Convention on the Law of the Sea (UNCLOS), adopted in 1982 and entering into force in 1994, is the foundational instrument of modern maritime law. Its provisions on international straits are unambiguous. Articles 26, 38, and 44 collectively establish that states bordering international straits may not suspend, block, or impede the right of transit passage through those straits, nor may they charge fees for such passage.

The right of transit passage, distinct from the more limited ‘innocent passage’ through territorial waters, applies to all ships and aircraft, and coastal states have no discretion to suspend it.

The Strait of Hormuz falls clearly within UNCLOS’s definition of an international strait. It connects the Persian Gulf to the Gulf of Oman and the Arabian Sea, linking the high seas on both ends. Multiple nations including the UAE, Qatar, Kuwait, and Bahrain have no alternative maritime route. The strait, for them, is existential.

Under UNCLOS, Iran’s closure of the strait, announced by the Islamic Revolutionary Guard Corps in late February 2026, confirmed on 2 March 2026, and enforced through warnings, mines, and attacks on vessels, constitutes a breach of international maritime law.

Iran’s legal position: a non-signatory’s dilemma

Here the picture becomes more complex. Iran never ratified UNCLOS. As a UN member state among 193 nations, it participated in the international system that produced the treaty, but withheld its signature.

Does this matter? The prevailing view among international legal scholars is that it does not, at least not entirely. UNCLOS is widely regarded as codifying customary international law in the maritime sphere. That means its core provisions, including freedom of transit passage through international straits, are considered binding on all states regardless of whether they have formally ratified the treaty.

The International Court of Justice has affirmed this principle in significant cases. In Nicaragua v. United States (Military and Paramilitary Activities in and against Nicaragua), the court held that customary international law binds states independently of treaty obligations, meaning non-signatories cannot escape rules that have been codified into treaty law simply by declining to ratify the relevant instrument. Iran cannot simply opt out of ‘freedom of navigation’; a principle that continues to be binding as part of customary international law,  merely by declining to sign the document that codifies the principle.

That said, Iran disputes this characterisation. Iranian officials have long argued that the strait runs through its territorial and contiguous waters, giving it sovereign rights over passage. This argument has not found broad acceptance in international law, but it illustrates the contested nature of the legal landscape.

The US blockade: freedom’s defender or rule-breaker?

The United States announced its own naval blockade of Iranian ports and coastal areas on 13 April 2026, after peace talks in Islamabad collapsed. The stated purpose is to prevent Iran from profiting through tolls it has been charging vessels for passage, and to restore freedom of navigation. US Central Command has stated that the blockade applies only to ships entering or leaving Iranian ports, not to vessels transiting the strait to third-country destinations.

This distinction matters legally. A total blockade of an international strait would itself violate UNCLOS and customary international law. The narrower scope claimed by the US, targeting Iranian commerce rather than international transit, is at least facially more defensible. But the legal questions remain significant.

The US, like Iran, has not ratified UNCLOS. Washington participated in drafting the treaty but declined to ratify it, primarily over disputes concerning deep-seabed mining provisions. Yet, also like Iran, the United States has historically relied on UNCLOS’s customary law status to assert freedom of navigation rights around the world, a posture it cannot easily abandon when the rules become inconvenient.

A naval blockade, even of a specific state’s ports, has well-established rules under the laws of armed conflict. It must be declared formally, enforced impartially, and must not cut off neutral states from essential goods. Whether the US blockade meets these standards, and whether an armed conflict legally exists between the US and Iran in the formal sense, remains contested.

A crisis without a referee

Perhaps the most troubling aspect of the Hormuz crisis is structural. On 7 April 2026, Russia and China vetoed a UN Security Council resolution aimed at protecting commercial shipping in the strait. 11 of 15 council members voted in favour; the two permanent members with veto power blocked it. The UN mechanism designed to enforce international law has, once again, been paralysed by geopolitics.

Without enforcement, the legal arguments, however sound, become largely academic. States that have not ratified UNCLOS cannot be dragged before the International Tribunal for the Law of the Sea. The ICJ requires consent to jurisdiction. With two great powers now in direct confrontation over the world’s most critical maritime chokepoint, the gap between law and reality has rarely been wider.

Conclusion

On the strict terms of international maritime law, both blockades are legally problematic. Iran’s closure of the strait to commercial traffic violates the right of transit passage enshrined in UNCLOS and reflected in customary international law. The US blockade, while more narrowly scoped, operates in legally uncertain territory, enforced by a non-signatory state, in a conflict without formal declaration, against a backdrop of broken diplomacy.

What the Hormuz crisis ultimately reveals is not merely a dispute between Iran and the United States. It is a stress test of the international legal order itself.

This article was co-written by Laura Stigaite, associate in the marine, trade and aviation team.