Hugo Grotius, the 17th-century Dutch jurist, would have found the Strait of Hormuz both familiar and dispiriting. In 1609, he published Mare Liberum (The Free Sea), arguing that the oceans belonged to no sovereign and that attempts to monopolise ocean routes were contrary to both reason and natural law.
His argument prevailed, in the long run. Four centuries later, it was codified in the United Nations Convention on the Law of the Sea, or UNCLOS, adopted in 1982 and now one of the most widely ratified treaties in history.
Grotius, however, did not have to reckon with the Islamic Revolutionary Guard Corps Navy or the United States Navy. And therein lies the problem.
What the law says
UNCLOS has been called the Constitution of the Oceans. Its 169 parties include Iran. The United States, with characteristic self-defeating exceptionalism, has never ratified it, though Washington vigorously enforces the rights it confers.
Under UNCLOS, the Strait of Hormuz falls within the regime of transit passage, a category with specific and important legal characteristics. It applies to straits used for international navigation, cannot be suspended by the bordering state, and covers all ships, including warships. In this case, Iran and Oman are the two bordering states.
Iran’s legal position, stated with varying degrees of aggression over the years, is that foreign warships require prior notification or consent before transiting, and that it retains broader sovereign rights than UNCLOS allows.
This position is, to put it charitably, legally weak. Iran ratified the convention. It is bound by the transit passage regime. Its contrary claims have no serious support in international legal scholarship. But Iran has never formally abandoned them, because legality and leverage are different currencies, and Iran trades primarily in the latter.
The geography of vulnerability
The stakes are not abstract. The strait connects the Persian Gulf to the Gulf of Oman and the broader Indian Ocean. At its narrowest, it is roughly 33 kilometres wide, with navigable shipping lanes of only a few kilometres in each direction.
Through this slender passage flows approximately 20% of the world’s oil supply and a substantial share of global liquefied natural gas. Saudi Arabia, the United Arab Emirates, Kuwait, Iraq and Qatar all depend on it for energy exports. Its closure would constitute an economic shock of global proportions – which is precisely what gives Iran its leverage.
Iran’s toolkit: harassment, mines, and the grey zone
Iran has not needed to close the strait to make its point. It has developed instead a layered repertoire of coercive action that falls just below the threshold likely to trigger overwhelming military response – what strategists call grey zone operations.
The most concentrated period of incidents occurred in 2019, after the Trump Administration’s withdrawal from the nuclear deal signed with President Barack Obama and the reimposition of maximum pressure sanctions.
The IRGC Navy seized, damaged and harassed tankers, causing insurance premiums to spike and shipping companies to reroute. These actions were clear violations of UNCLOS and international maritime law. The response was diplomatic protest and naval escorts, not legal accountability.
The seizure of the British-flagged tanker the Stena Impero in July 2019 was particularly instructive. Iran held the ship for more than two months. The legal position was straightforward: seizing a foreign ship in transit passage violates international law. The tanker was eventually released through diplomatic negotiation. No legal remedy was obtained. Iran faced no accountability before any international tribunal.
Legality and consequences, it turned out, were not the same thing.
The Houthi dimension and the American paradox
From late 2023, the conflict in Gaza added a troubling further dimension. Yemen’s Houthi movement, backed by Iran, began attacking commercial shipping in the Red Sea and Gulf of Aden, forcing operators to reroute around the Cape of Good Hope.
The Houthi campaign illustrated the reach of Iranian strategic depth through its network of proxy forces developed across the Middle East. The Hormuz challenge had extended into adjacent waterways.
The US and the United Kingdom conducted strikes against Houthi targets in Yemen, operating outside any UN Security Council authorisation and framing the action as collective self-defence of freedom of navigation.
Russia and China opposed or criticised the strikes. Great power rivalry fractured any coherent institutional response, as it invariably does when permanent members’ interests diverge.
What the strait teaches us
The Strait of Hormuz is, in miniature, the entire rules-based international order. Every tension, every structural weakness, every gap between legal principle and practical enforcement is visible there with unusual clarity.
UNCLOS provides clear rules. Iran formally violates them. No court can compel compliance. The practical openness of the strait rests not on legal entitlement but on the continuous presence of the US Navy in the Persian Gulf. Now that presence reverses access to become restriction.
Law and military deterrence are not alternatives here; they are complements. The law provides the language and framework; the fleet provides the consequence. Remove one, and the other becomes a pious aspiration.
What has arguably constrained Iranian behaviour more than either law or warships is the blunt reality of economic interdependence. China imports enormous quantities of Gulf oil through the strait. Beijing has no interest in its closure and has, at times, exercised quiet influence accordingly.
The shared economic interest of major powers in keeping the strait open has provided a form of order that the legal framework alone could not supply. Grotius would have recognised this: his argument for freedom of the seas was ultimately commercial as much as it was legal.
The consequence of failure
The exercise of hard military power in the region has not succeeded in keeping the Strait of Hormuz securely open. Operation Epic Fury, whatever its other aims, does not appear to have secured strategic control of the strait. Iran’s grip, far from being loosened, appears to have tightened. The US Navy now also applies power.
Of all the miscalculations that may attend great power competition in the Gulf, this may carry the longest shadow. The strait handles a fifth of the world’s oil. The consequences of Iranian dominance over it – the ability to tax, threaten, harass, or close the passage at will – extend to every economy that depends on affordable energy, including New Zealand’s.
Grotius argued that the sea could not be owned. He was right in principle. But principles require enforcement, and enforcement requires will. The rules-based order did not fail at Hormuz because the rules were wrong. It may be failing because those with the power to enforce them miscalculated, divided, or simply looked away.
The free sea, it turns out, is only as free as the willingness to keep it so.
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