Iran has said it will reopen the Strait of Hormuz if certain conditions are met. That raises a question the coverage tends to skip: is a country allowed to close a strait at all?

The short answer is no. The longer answer explains why that matters less than you would hope.

Whose water is it

Under the law of the sea, a coastal state controls a belt of water extending twelve nautical miles from its shore — its territorial sea. Within that belt it has sovereignty, subject to certain rights held by other states.

The Strait of Hormuz is narrow enough that there is no corridor of international water down the middle. Every ship passing through is in the territorial sea of Iran or of Oman. That geography is the entire reason the strait is a pressure point.

The rule for straits

Because that situation arises in many places, the law treats straits used for international navigation as a special case. Ships and aircraft have a right of transit passage: continuous and expeditious passage through, which the bordering state may not suspend.

That last part is the key. Ordinary innocent passage through a territorial sea can be suspended temporarily. Transit passage through an international strait cannot. The drafters recognised that a handful of countries would otherwise hold a permanent switch over the world’s trade.

So closing the strait is not a permitted act. There is no legal reading under which it becomes one because the closing state has grievances, however genuine.

Where these rules came from

The current framework was not handed down from anywhere. It was negotiated over years, and the straits question was among the hardest parts of it.

The difficulty arose from an earlier change. When states extended their territorial seas from three nautical miles to twelve, dozens of waterways that had contained a corridor of international water suddenly did not. Overnight, a large number of major shipping routes fell entirely within somebody’s territorial sea.

Maritime powers found that unacceptable. Coastal states were unwilling to give up the extension. Transit passage was the compromise: coastal states kept the wider territorial sea, and in exchange passage through international straits was made non-suspendable.

Understanding it as a bargain rather than a principle explains why it is both firm on paper and contested in practice.

Other straits, other arrangements

Hormuz is not the only chokepoint, and the world’s narrow waterways are governed in strikingly different ways.

The Turkish Straits, linking the Black Sea to the Mediterranean, are governed by their own long-standing convention that gives Türkiye specific powers over warships, particularly in wartime — a regime older than the modern law of the sea and preserved alongside it.

The Suez and Panama canals are different again: artificial waterways under the control of a single state, governed by treaties guaranteeing access, and operated commercially with tolls.

The Strait of Malacca, carrying an enormous share of Asian trade, is bordered by three states that have cooperated on patrols and safety — an example of a chokepoint managed largely without confrontation.

The pattern is instructive. Where bordering states benefit from traffic, arrangements tend to hold. Where a state sees leverage in obstruction, no legal text prevents it from trying.

Why the rule does not settle it

Three complications.

Enforcement. International law has no police force. A breach is answered by other states — through protest, sanctions, escorts or force — and each of those is a political decision with its own risks.

Nobody has to announce a closure. A strait becomes impassable in practice when shipowners and insurers judge it too dangerous. Attacks on a few vessels can achieve that without any legal act to point to.

Ratification. Not every state is party to the convention, and some that are have entered their own interpretations. Iran signed but has not ratified it, and disputes parts of the transit passage regime — though much of the law of the sea is also treated as binding customary law regardless.

What the law is actually for

It would be easy to conclude the rules are decorative. That reading is too quick. The existence of a clear legal position shapes what other states can justify doing in response, gives neutral countries a basis for objecting, and makes the closing state’s position harder to defend internationally.

Law rarely stops a determined state on its own. It determines the cost of acting, and who is willing to stand alongside you afterwards.

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Background: United Nations Convention on the Law of the Sea, Part III (straits used for international navigation).