TEHRAN - An examination of the legal grounds for the US blockade of Iranian ports, what the UN Charter says, and how international law experts assess the action.

Imagine a country deciding to stop another country’s commercial ships, prevent goods from reaching its ports, and effectively impose its own rules on vessels travelling through international waters. Does military power, combined with claims of national security, give that country the right to do so? If the answer is yes, what would stop any major power from turning the seas into tools of pressure and blockade whenever it has a dispute with another country?

These questions lie at the heart of the US naval blockade of Iran. A naval blockade is not simply a matter of stopping ships. It can disrupt a country’s trade, drive up economic costs, and affect people’s daily lives. For that reason, international law does not give countries unlimited authority to restrict maritime traffic simply because they have military superiority or claim to be acting for security reasons.

Under international law, the US naval blockade of Iran cannot be justified simply by Washington’s announcement or its claims of national security. Given the absence of UN Security Council authorisation and the lack of a valid legal basis for self-defence, the blockade, as part of the use of force against Iran, is illegal. One important distinction must be made, however: even during wartime, rules govern how a naval blockade may be carried out. Possible compliance with some of those rules does not, by itself, make the original use of force lawful.

What does the UN Charter say?

The main legal document relevant to the US action is the United Nations Charter. Article 2(4) prohibits states from threatening or using force against other states. The purpose of this rule is to prevent governments from using their military strength to impose their will on other countries.

The Charter recognises two principal legal grounds for the use of force: authorisation by the UN Security Council and self-defence under the conditions set out in Article 51. A country seeking to justify a military blockade of another country must therefore demonstrate that its actions fall within one of these legal frameworks.

In the case of the blockade of Iran, the United States cannot create its own legal authority simply by declaring a security emergency or seeking to put economic pressure on Iran. If the military action lacks a valid basis under the UN Charter, it cannot become lawful merely because Washington gives it an official name or announces a justification.

What do international law experts say?

One important contribution to the legal debate is an article by Michael Schmitt, a professor and specialist in international law, published by Just Security. In his article, “Iran’s Conditions for Reopening Hormuz: What International Law Requires, Permits, and Forbids,” Schmitt examines the legal basis for the US use of force against Iran.

Schmitt explains that, under the UN Charter, the use of force must generally be based on Security Council authorisation or the right of self-defence in response to an armed attack. According to his analysis, most international law experts believe that the initial US attack was not authorised by the Security Council and did not meet the necessary conditions to qualify as self-defence. He therefore considers the start of US military operations to be inconsistent with the country’s obligations under the UN Charter.

His analysis is directly relevant to the blockade. If the legality of the original use of force is in question, the blockade cannot be treated as an entirely separate action and declared lawful simply by referring to the rules of naval warfare.

Steve Prest, a retired naval officer and analyst at the Royal United Services Institute (RUSI), raises similar concerns in his article, “The ‘US Blockade’ of Iran: How It Might Function (Or Not!).” He argues that the legality of the US action depends on whether the necessary conditions for using force have been met. Without Security Council authorisation, self-defence is the principal legal justification available to Washington, but Prest points to serious questions about whether that argument can justify the operation.

These analyses highlight the central issue: the question is not merely how the United States stops ships, but on what legal grounds it claims the right to use military force to blockade Iranian ports.

Do the laws of war make the US blockade legal?

The United States may argue that naval blockades are a recognised method of warfare. That claim reflects part of the legal picture, but it does not tell the whole story.

The San Remo Manual on International Law Applicable to Armed Conflicts at Sea, adopted in 1994, sets out restrictions on naval blockades. Under its rules, a blockade must be declared, its limits must be specified, and it must be effectively enforced. It must also respect the rights of neutral states. Rules protecting civilians and ensuring access to essential supplies must be observed as well.

The San Remo Manual is not, in itself, a binding treaty. However, it is a recognised reference for assessing the law governing naval warfare.

The key point is simple: the fact that certain methods of warfare may be lawful does not mean that starting a war or using force is lawful.

A legal analysis published by German broadcaster Tagesschau on 18 July 2026 makes the same distinction. Citing Pierre Thielbörger, a professor of international law at the University of Bochum, the report explains that the legality of resorting to force must be distinguished from the legality of how military operations are conducted. In other words, a particular action may be assessed separately under the rules of naval warfare, but that does not erase the possibility that the war itself was unlawful.

For this reason, citing the rules governing naval blockades does not, on its own, answer the fundamental question: did the United States have the legal right to use force to blockade Iranian ports in the first place?

US sanctions do not give Washington the right to shut down Iran’s trade

Another issue is the attempt to link the naval blockade to US sanctions. Washington may place companies or ships on its sanctions lists, but a decision made by one government does not automatically become international law.

Keri Batsov, a former US Treasury Department official, said: “Iran will not surrender under sanctions; the naval blockade has worn down the US military, and Iran’s low-cost drones have challenged Washington’s expensive weapons.”

Article 92 of the UN Convention on the Law of the Sea recognises the principle that ships on the high seas are generally subject to the jurisdiction of their flag state. Article 110 sets out specific circumstances in which foreign ships may be boarded on the high seas.

These provisions make clear that the seizure of ships in international waters requires a legal basis. A vessel’s commercial links to Iran, or its inclusion on a US sanctions list, does not by itself give the United States unlimited authority to seize it.

Mark Nevitt, a legal scholar specialising in military and maritime law, raised concerns about this issue in an article published by Just Security on 1 May 2026. He warns that the United States is grouping several types of maritime operations, each with a different legal basis, under the single label of a “blockade.” He distinguishes a blockade of Iranian ports from the seizure of US-sanctioned ships in different parts of the world, stressing that each operation must have its own clear legal justification.

This distinction is crucial. Even if a limited blockade under certain conditions of armed conflict can be defended on legal grounds, that does not give the United States the authority to stop every ship linked to Iran anywhere in the world.

The human cost of a naval blockade

A naval blockade is not just a military measure directed at ports. Its effects can reach deep into the economy and people’s everyday lives. Restrictions on ships entering and leaving ports can disrupt trade, raise shipping costs, and make essential goods more difficult to obtain.

International humanitarian law contains rules intended to prevent unjustifiable harm to civilians. Using starvation of civilians as a method of warfare is prohibited, and a blockade must not be used as an excuse to disregard people’s essential needs.

Ismail Kowsari, an Iranian parliament member and member of the National Security and Foreign Policy Commission, has called for legal action over US measures against Iran. He said international legal mechanisms lack sufficient power and enforcement guarantees to address violations and hold those responsible to account. This, he said, has allowed some powers to pursue their interests without sufficient regard for the rights of other countries and peoples.

Kowsari said it was necessary to stand firmly against this approach and to drive the United States out of the region so that international law could be implemented in practice. Criticising international judicial institutions for failing to address US violations earlier, he said this should not bring legal efforts to a halt. Iran’s judiciary, he argued, should carefully document US actions and violations against Iran, including the naval blockade, and pursue them through legal channels.

Kowsari also stressed the need to document the humanitarian consequences of sanctions, particularly in the medical and pharmaceutical sectors. He said patients and vulnerable groups depend on essential medicines and medical equipment, and that any disruption to access could have serious consequences. Despite these needs, he added, sanctions can obstruct access to essential supplies while those imposing them continue without sufficient regard for the human impact.

He said the effects of these measures should be recorded accurately through credible documents, evidence, and other supporting material so that they can be pursued before the competent international bodies.

Assessing a blockade therefore requires more than counting the number of ships stopped or measuring the economic pressure it creates. Its impact on civilians must also be examined, including whether essential supplies remain available in practice.

A naval blockade can thus become more than an instrument of military pressure. It can develop into a humanitarian and legal crisis, and governments cannot escape responsibility simply by declaring that their actions serve security objectives.

The UN Charter and the analyses of international law experts show that the blockade of Iran cannot be assessed solely through the technical rules of naval warfare. The fundamental question remains: on what legal basis is the United States using force to blockade Iranian ports?

Michael Schmitt, writing for Just Security, questions the legal basis for the initial US attack. Steve Prest, writing for RUSI, raises serious doubts about whether self-defence can justify the operation. Mark Nevitt, also writing for Just Security, explains why a limited blockade cannot be turned into a blanket licence to seize sanctioned ships around the world. The UN Charter sets out the underlying principle: the use of force must be justified under international law.

The US naval blockade of Iran is therefore illegal in the absence of Security Council authorisation and a valid legal basis for the use of force. The rules of naval warfare cannot, on their own, resolve this fundamental problem. Nor do unilateral US sanctions give Washington unlimited authority to control Iran’s maritime trade.

This is not just about Iran. It is about whether major powers will abide by the rules they accepted as part of the international order established after the Second World War. If military power is allowed to replace the law, naval blockades will no longer be exceptional measures. They could become tools that powerful countries use whenever they want to force another state to submit to their demands.