The ‘US blockade’ of Iran: How It Might Function (Or Not!)

On 12 April, President Donald Trump declared that the US would blockade the Strait of Hormuz. This was in response to the collapse of diplomatic talks in Islamabad, themselves precipitated by a two-week ceasefire agreement in the ongoing hostilities between the US and Israel, and Iran. Later that day, US Central Command then clarified the Trump’s announcement in their formal declaration of a blockade of ‘all maritime traffic entering and exiting Iranian ports’ but stressed that Freedom of Navigation would not be impeded ‘for vessels transiting the Strait of Hormuz to and from non-Iranian ports.’

A Strategic Shift

Such a move represents a shift in strategy from the US with regard to its conflict with Iran. There is no doubt that the might of the US Armed Forces has been brought to bear with devastating effect against large parts of Iran’s military, political and industrial infrastructure. Militarily it has been a largely one-sided fight and although Iran has landed a number of blows, these are nothing compared to the pummelling that it has received from the US and Israel.

Nonetheless, the Iranians have long prepared for such an eventuality and have not sought to go toe-to-toe with the US on the seas or in the air. They have dispersed, hidden and dug-in much of their capability to make it both hard to find and hard to strike. It is not known how much remains, but no one thinks that the Iranian ability to make mischief in the region has been eliminated.

This has enabled Iran’s continued ability to strike a range of targets in the Gulf region with drones and ballistic missiles, the targets of which have included US bases as well as the economic interests of their neighbours. The aim, whilst seeking to denude what they can of the US capability, is to apply diplomatic and economic pressure both directly and indirectly to the US to bring an end to hostilities.

The Iranian trump card (no pun intended) is the Strait of Hormuz. This narrow stretch of water links the Persian Gulf to the Gulf of Oman and in normal times it sees the flow of around 20% of the world’s hydrocarbon supplies. Since the outbreak of hostilities, Iran has effectively closed the Strait to those ships whose passage it does not see as in its interests. It has continued to allow ships through to export some of its own oil, and some other limited traffic, but the maritime traffic flow has been a small percentage of the previous volume.

This restriction of supply has had a predictable inflationary effect on oil prices and certain other important commodities. Fertiliser is another such product, the restriction of which during the northern hemisphere’s Spring (and thus planting season) may well bake-in food inflation later in the year. It is this stranglehold on the World Economy that Iran hopes will impose such economic and political costs that their adversaries will be forced to ‘tap out’ and cut a deal.

For now, at least, the US Administration is showing little appetite for returning to a full-scale assault from the air nor, it seems, has it the willingness to commit amphibious and ground forces. Rather, it is, itself, concentrating its military to apply strategic economic leverage. By blockading Iranian ports, it is seeking to destabilise the Iranian economy and bring the Iranian regime to the point of capitulation. War is an extension of politics by other means.

Is it Legal? Does it Matter?

The legality of such an action is dependent on whether or not a declared state of war exists between the two countries. Under International Law a wartime blockade must be formally declared and notified and enforcement must be applied against all shipping, including those of neutral and belligerent countries alike. This is consistent with the CENTCOM declaration. Notwithstanding that under the US Constitution only Congress can formally (legally) declare war, and it has not done so, Iran and the US can be considered as at war as far as the application of the Law of Armed Conflict is concerned. An international armed conflict is clearly underway between them.

Under the United Nations Charter, to which both the US and Iran are signatories, the only legitimate reason to undertake such activity, in the absence of a UN Security Council resolution on the matter, is in self-defence. The case for self-defence has been asserted by the US but no serious effort seems to have been made to make the argument that the legal threshold for such action has been met.

Of course, such arguments are applicable to the US intervention in Iran as a whole and, whilst such legal niceties seem to be of little concern to this Administration, it would be a real stretch to try and argue that oil shipments posed any such threat in and of themselves. This is concerning for those who consider the rules-based international order to be important. It would appear this includes, in a bizarre inversion over issues of Freedom of Navigation, the Chinese Premier who on 14 April said ‘The world faces a contest between justice and power,' warning against a return to the ‘law of the jungle.’

So, How is it Going to Work?

The first thing is that much of the maritime traffic that might have been expected to ply their trade from Iranian ports will, most likely, be deterred by the declaration of the blockade in the first instance. Even where ships might previously have been content to trade with Iran during such hostilities, the risk of being interdicted and impounded by the US Navy will be beyond the risk threshold for many commercial operators.

Nonetheless, there may well be some who are tempted to run the risk. So, what happens then?

The Middle East is the ‘patch’ of the US Fifth Fleet, and they will already have a comprehensive intelligence picture of merchant shipping traffic in the area. The challenge will be to track those who seek to disguise their movements. That is to say, those whose Automatic Identification System (AIS) is providing a report of the ship’s location and movements different to the reality. For example, AIS could be manipulated to say the ship was at anchor somewhere in the Northern Gulf when, in fact, they had slipped across to Kharg Island to fill up their tanks. The open-source data would show that they had never moved, but in reality they are now running the blockade. Especially perplexing if they then enter a third-nation’s port before trying to exit the Strait. How many port visits ago does a visit to Iran have to have been for it to count?

There is also a possibility, of course, that such blockade running could take place using ship-to-ship transfers in international waters without the ship then transiting the Strait ever having been inside Iran’s territorial waters. Of the ships that are reported as having passed through the Strait on 14 April, at least some may fall into one of these categories and (at time of writing) have not yet been intercepted.

All of this will need to be tracked, assessed and decided upon before any action is taken. The assets to do this will range from cyber, satellites, drones and AI-supported intelligence analysis through to maritime patrol aircraft, crewed warships, naval helicopters and commandos. The main surveillance activity will take place in the Persian Gulf itself, to track the activity of the ships therein, and the Iranian Coast on the Gulf of Oman. Any interdiction of shipping is likely to be further out in the Gulf of Oman or Arabian Sea, keeping the US Navy ships out of reach of coastal missile and drone batteries. This area of ocean, however, is about 250,000 square miles. It will tie up several warships to maintain a credible blockade.

Assuming that a suspected blockade runner is to be interdicted, a ship or ships will need to be allocated to conduct that operation. Most ships will simply be turned away by the US Navy, either from Iran or back to Iranian ports.

It may be, however, that a boarding team is to be embarked onto the target ship. Normally this would be led by commandos who would fast-rope down from a helicopter or climb a ladder from a boat. Whilst relatively routine, neither option is without risk. Ships will be expected to be compliant – it is highly unlikely that such boardings would be opposed by a civilian crew. That said, if the flag nations of the vessels, such as, say, China, decided to escort their tankers out of the area with warships of their own, or embark military detachments onboard the merchant vessel, then the problem becomes far more complex. It is highly unlikely the US Navy would attempt to board vessels with such an escort. We have seen a similar dynamic with Russian shadow fleet vessels, escorted by Russian Navy units, in Northern Europe.

But boarding may not be required. A radioed instruction to divert the ship to anchorage or to a harbour under escort may be sufficient. But to where? It is unlikely that Oman would want to be the repository for such vessels – that would put them in a very awkward position with their neighbours. The same is true for Pakistan. It is hard to see anywhere else nearby that might be suitable. And even if you find such a port, what then? Do you seize the vessel and/or the oil for your own as spoils of war? Impound it until hostilities have ceased and then let it on its way? Or something else? These are questions I have not yet heard discussed or answered in the discourse.

To conclude. The strategic switch of the US to targeting Iran’s petrostate economy directly is a smart move. It is likely to destabilise the regime in the way that air bombardment has not. It does not preclude a return to the more direct application of violence, but it will be cheaper and less risky for the US. There are practicalities that need to be worked out, but these are not insurmountable in the execution of the blockade. The biggest risk, however, is from escalation either directly or indirectly with some of the big economies (such as China) that will now come under increased pressure as a result of this action. It remains to be seen how they will respond but, if the impacts start to bite, respond they will. We live in dangerous times.

© RUSI, 2026.

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