Was the US seizure of an Iran ship legal? Depends if this ceasefire is considered as war or peace

So who’s right here? The answer depends on how the larger scenario around the episode is interpreted: Is this wartime or does the ceasefire mean this is peacetime?

Here’s what international law says about such maritime conflicts.

What’s the status of the conflict?

The US-Israeli alliance began the war on Iran on February 28. Iran responded by effectively “closing” the Strait of Hormuz, the narrow passage that accounts for a fifth of the world’s energy flows in peacetime. A temporary ceasefire took effect on April 8, but, days later, US President Donald Trump announced a naval blockade aimed at restricting Iran’s oil exports and intercepting vessels entering or exiting its ports. Iranian forces retaliated by intercepting foreign-flagged oil tankers on April 18 — two days before the commencement of the second round of diplomatic talks scheduled in Islamabad.

In this crisis, the overriding legal question concerns the “status” of the conflict and the exact geographic coordinates of the US blockade, the details of which remain unknown.  Unlike domestic law, international law lacks a single global police force; its enforceability relies on treaties, customary practices and the reality on the ground. The 1982 UN Convention on the Law of the Sea (UNCLOS) regulates peacetime activities. International conflicts, meanwhile, are governed by the Law of Active Combat (LOAC), which is also known as International Humanitarian Law (IHL). The LOAC says an ‘international armed conflict’ exists whenever there is a “resort to armed force between States”, regardless of whether a formal declaration of war has been made. This is where the transition from a ceasefire becomes legally tricky. A ceasefire is merely a pause in fighting, not a legal end to a war.

The US may view the underlying February conflict as still active and argue, therefore, that imposing a blockade is a lawful resumption of hostilities. Iran, however, may argue that the ceasefire functioned as an end to the fighting, making the blockade an unprovoked attack in peacetime.

So what does international law say?

A naval blockade is strictly a wartime operation, according to maritime law. While the main conditions regarding the status of the conflict and exact coordinates of the blockade determine the legality of the action, the multiplicity of doctrinal choices in maritime law must also be acknowledged. There is the modern LOAC, which is underpinned by the Geneva convention, the San Remo manual (1994) and international Prize law (whose formal domestic courts have been largely dormant since World War II). There is also the UN Charter, whose Article 2(4) prohibits member states from using or threatening force against the territorial integrity or political independence of any state.

Should it be determined that the US and Iran are in an active state of armed conflict, the rules of peacetime are immediately suspended, and the laws of naval warfare apply. The legality of the US operation hinges on which framework is currently applicable to the situation:

Peacetime: Had the April 8 ceasefire been legally binding and dictated terms for the restoration of peace, the US actions would be definitively illegal. The governing law would be the UNCLOS, a peacetime treaty guaranteeing commercial ships the “freedom of the high seas”. UNCLOS does not grant foreign militaries the “right of visit” to forcibly board a ship merely to enforce unilateral economic sanctions. Through this lens, firing upon the Touska would be a violation of Iranian sovereignty. While Tehran has termed this as “piracy”, UNCLOS Article 101 states that violent action taken to meet private ends is termed as piracy and cannot be designated to sovereign agendas. 

Wartime: If the ceasefire collapses, UNCLOS evaporates, and the US invokes the LOAC. Here, the following doctrines could be applicable:

The San Remo Manual: While the Geneva conventions protect individuals (such as wounded sailors), San Remo is the universally recognised code governing naval warfare and the capture of property at sea. Paragraphs 118 and 135 of the manual grant ‘belligerent’ warships the right to visit and search merchant vessels while providing provisions for the capture of enemy-flagged vessels. 

Prize law: The US capture of the Touska within the San Remo guidelines is complemented by Prize law, a historical law of the sea. It allows Washington to bring the Touska before a domestic Prize court to legally transfer the title of the ship and its cargo to the US. But the last time a US Prize court convened was around the time of World War II.

US doctrine: The San Remo Manual also underpins the US Navy’s Commander’s Handbook on the Law of Naval Operations. The handbook dictates that enemy merchant vessels “may be captured wherever located beyond neutral territory”. Assuming that the war remains active, Touska loses its civilian immunity and can be reclassified as an “enemy merchant vessel”. While neutral vessels are bound to stop for a search, the US Handbook explicitly notes that an enemy ship “is not bound and may legally resist”. However, by refusing to stop, the enemy vessel “assumes all risk of resulting damage”. By actively ignoring the USS Spruance’s warnings for six hours, the Touska transformed from a protected civilian object into a valid military target.

However, the handbook holds no legal weight in a peacetime scenario. Its aggressive rules of engagement only become internationally valid if the US successfully argues that the April 8 ceasefire failed to terminate the overarching international armed conflict. 

The way ahead

The US Navy’s actions appear carefully calibrated to adhere to the wartime rules of “proportionality” and “precaution”. By warning the crew to evacuate the engine room and opting to disable propulsion rather than sink the vessel, the US minimised civilian casualties and adhered to the rules of naval engagement as set down by the Geneva Convention. 

This brings us back to our first question. Are the US blockade and seizures legal? Yes, if it is indeed deemed that the US and Iran continue to remain in a state of armed conflict. And no, if the current ceasefire is interpreted as peacetime, in which case the seizure is an act of piracy under the UNCLOS.