14 April 2026
“Under the law of naval warfare, a blockade is only lawful if it meets strict requirements"
As the US Navy starts a blockade on Iranian ports, Associate Head of the University of Sunderland’s Law School, Associate Professor Pardis M Tehrani, asks – what is the legal basis for any attempt by the US to control or restrict one of the world’s most strategically important maritime chokepoints?
“The Strait of Hormuz occupies a central position in both geopolitical discourse and the law of the sea. The legal status of the Strait raises fundamental questions regarding the balance between state sovereignty and the international community’s interest in freedom of navigation. Part III of the United Nations Convention on the Law of the Sea (UNCLOS) establishes a special regime of transit passage that must not be impeded, while coastal states such as Iran retain sovereignty over their territorial sea.
However, this sovereignty is subject to the right of transit passage. Transit passage, codified in UNCLOS Articles 37–44, entails continuous and expeditious passage, freedom of navigation and overflight, applicability to all vessels, including warships, and is non-suspendable.
A key point is that Iran, the United States, and Israel are not parties to the UNCLOS; however, this does not negate the applicability of the transit passage regime, which is widely regarded as part of customary international law, based on consistent state practice and opinio juris – a belief that states are legally required to follow it. As such, it is binding on all states, including Iran, irrespective of treaty participation. Iran’s position that it may condition passage or require prior authorisation for warships, therefore, lacks strong doctrinal support.
The right of transit passage has been recognised and developed through cases such as Corfu Channel, Nicaragua v United States, and the Oil Platforms case, decided by the International Court of Justice. Therefore, the imposition of financial conditions (including tolls), selective access, and mandatory coordination with authorities is inconsistent with the core principles of international law, including the prohibition on hampering transit passage (UNCLOS Article 44), the non-discrimination requirement, and the prohibition on suspension.
Another legal argument Iran may invoke is the right of self-defence under Article 51 of the UN Charter, arguing that the closure is a lawful response to armed attack by the US and Israel. However, this right is not unlimited; it is restricted by the customary international law requirements of necessity and proportionality, as set out in the Nicaragua case, which held that only measures that are necessary and proportionate in response to an armed attack are lawful in self-defence. Iran’s mining of the Strait of Hormuz complicates the situation because it unlawfully restricts transit passage for neutral shipping, in violation of both the law of the sea and the law of naval warfare.
Following reports of President Trump’s announcement regarding a blockade of the Strait of Hormuz, CENTCOM (US Central Command) later clarified that the operation concerned restrictions on access to Iranian ports rather than the Strait itself.
"Under the law of naval warfare, a blockade is a belligerent act attributable to a state in armed conflict, and is only lawful if it meets strict requirements, including declaration, effectiveness, impartiality, proportionality, and respect for humanitarian and neutral rights. Its legality, therefore, depends on whether these conditions are satisfied in practice.
The coexistence of these regimes leads to normative fragmentation, allowing states to selectively justify their conduct. In my view, the Strait of Hormuz shows that while international legal rules are clear, their effectiveness ultimately depends on political will and state behaviour.”
Associate Professor Pardis M Tehrani is available for interview.