With peace negotiations between the United States and Iran breaking down in Pakistan, the fragile ceasefire is again on the brink of collapse. US President Donald Trump has again upped the war rhetoric, saying the US navy will now blockade the Strait of Hormuz.The president said the US would intercept ships that had paid Iran a toll to pass through the strait, accusing Iran of “extortion” in charging the fee.However, the New York Times is reporting the US military plans to block only Iranian ports and coastal areas, while allowing other vessels to pass through, which appears to be a softening of the president’s initial statement. It reported the blockade would begin on Monday morning, US eastern time.So if such a blockade goes ahead, what would it mean and what implications would it have?In warfare, a blockade is legalAt the moment we are in something of a holding pattern in the US-Israel-Iran war. Peace talks were underway – though have since collapsed – but there is no active force being used at the moment. However, there is also not a settled peace.So the critical question is whether the US or Iran is engaging in any conduct that imperils a ceasefire – and US naval ships blockading the Strait Hormuz, a vital economic passage, would certainly do that.In purely legal terms, if the US imposes a blockade then the ceasefire is over and hostilities have resumed. Since the most recent hostilities commenced on February 28, Iran has effectively been prohibiting navigation in the strait. Iran’s conduct can be justified on the grounds that an armed conflict has been underway and the safety of all shipping passing the through the strait has been under threat. Some ships have passed through the strait either under Iranian escort, or reportedly after having paid a form of “toll” to Iran. This conduct needs to be understood in the context of an international armed conflict where normal freedoms of navigation are suspended.It’s also important to make a distinction between the actors in this scenario. In international law, we talk about “belligerents” and “neutrals” in terms of war. There is no question the US, Israel and Iran are belligerents in this case. So this means the US, as a belligerent, can legitimately impose a blockade under international law – specifically, the law of naval warfare. There is a long history of countries imposing maritime blockades as a method of warfare. The most recent example was in December 2025, when the US imposed a blockade on oil tankers travelling in and out of Venezuela, in addition to placing sanctions on the country. Given how vital the Strait of Hormuz is to the passage of crude oil that is vital for the world’s fuel supply, a blockade would further limit this supply and send petrol prices soaring even higher. Trump has also threatened China with 50% tariffs if it is found to be assisting Iran in the war.No clear path forwardIt is not yet clear exactly how, and in what form, this blockade will be imposed, or even if it will happen at all.Trump’s initial claim that the US military would block the entire strait, and pursue vessels that had paid money to Iran to be allowed through, would cause further diplomatic ructions with countries around the world. The president said other countries would be involved in the blockade, but Prime Minister Anthony Albanese said Australia had not been asked to be involved and he did not expect it to be.However, if the intention is to limit the blockade to Iranian ports then that would be a very traditional form of naval blockade between “belligerents”. Ships that have not come from Iranian ports and that so far have been barred from exiting the Persian Gulf through the strait could then conceivably start to move through the strait and into the Indian Ocean.If the blockade does go ahead, it will demonstrate the enormous capability of the US military, given it will require the deployment of serious naval assets. It will also confirm Trump’s claim the US has control of the area.It is, too, a powerful negotiating tactic, because of the economic hardship it will impose on Iran if it is unable to continue exporting oil. This will only add to the intense suffering of the Iranian people. The freedom of navigation through international straits like Hormuz is one of the core principles of international law on which international trade and the global economy rests. It is one the US has historically staunchly defended. The threatened blockade of Iran and associated mine sweeping operations in the strait may be one small step towards restoring the Strait of Hormuz as a maritime highway open for all to use. Major Iranian oil ports, terminals and refineries. Reuters, CC BY-SA Donald Rothwell receives funding from Australian Research Council