International Law: Who Is Allowed to Sail Through the Strait of Hormuz

Professor Schatz, how does the current situation in the Strait of Hormuz fit into the framework of international law?

As a strait, the Strait of Hormuz consists entirely of the territorial seas of two states at a crucial point, requiring passage through Iran’s territorial waters. Territorial waters are not considered international waters but rather waters that are fundamentally subject to the sovereignty of the coastal state. Normally, however, ships are permitted to pass through territorial waters under the principle of innocent passage.

What exactly does “peaceful passage” mean?

This means that both merchant and warships are permitted to pass through, but only under certain conditions. Warships must not engage in hostile behavior, such as conducting military exercises or using weapons. Other activities are also prohibited, such as fishing or disposing of waste without permission. Submarines, for example, must travel on the surface.

Are there special rules for straits?

Yes. For straits that are of particular importance to international shipping, maritime law provides for the right of so-called transit passage. This goes beyond peaceful passage and cannot be suspended by the coastal state—not even for reasons of national security.

Does this right also apply in the Strait of Hormuz?

That is a matter of dispute. Strictly speaking, Hormuz would be a classic case for transit passage. However, Iran is not a party to the United Nations Convention on the Law of the Sea. Therefore, there is debate over whether this right also applies as customary law. The U.S. holds this view, but is itself not a party to the convention either. Both sides thus selectively invoke certain rules. There is at least agreement that a right of peaceful passage applies, and according to the prevailing view, one that cannot be unilaterally suspended by the coastal state.

There is repeated talk of possible fees, i.e., a kind of toll, for passage. Is that permissible?

No, not in principle. No money may be charged for mere passage—regardless of whether a right of innocent passage or transit passage exists. This is a very old principle of the law of the sea. The only exception is when specific services are provided, such as pilotage services in dangerous waters. However, this can hardly be justified for the Strait of Hormuz. Unilateral imposition of fees would therefore be impermissible under international law.

What changes in the event of war?

The belligerents—such as Iran, Israel, and the United States—are each permitted to attack military targets. This also applies to warships. Merchant ships flying the flag of a belligerent may also be captured under the so-called law of prize, assuming that this law can still be classified as applicable customary international law today, despite the fact that it is now only sporadically applied in state practice. However: Neutral ships—that is, ships flying the flags of states that are not belligerents—may continue to pass through. They may neither be attacked nor simply detained. Therefore, a complete blockade of the Strait of Hormuz would not be permitted under international law.

The U.S. is conducting a targeted naval blockade against Iran. What does this mean legally?

A naval blockade is an instrument of the law of naval warfare and is classified as economic warfare. It is not directed against a strait itself, but always against a state. If it is lawfully imposed, ships en route to the blockaded state may be stopped and inspected. Only ships transporting medical supplies or food necessary for the population must, in principle, be allowed to pass for humanitarian reasons, although there are differing legal opinions on this point. If ships attempt to break through a blockade, they may be boarded or, if they resist, attacked as a last resort after a prior warning.

It is important to note that a blockade must, among other things, be announced, clearly defined in terms of time and space, and enforced consistently. Such classic naval blockades are rare today, which is why current developments are of particular interest from the perspective of international law scholarship.

Leuphana offers two master’s programs with a focus on international law. Why is this topic so relevant today?

In the current geopolitical situation, some actors are increasingly challenging the existing institutional and legal order. Nevertheless, recent cases such as the Strait of Hormuz demonstrate very concretely how strongly certain rules of international law continue to shape global developments and the behavior of affected states—especially when these regulations form the backbone of global maritime trade. Especially in crisis situations—such as the current one in the Strait of Hormuz—states and the maritime industry must therefore receive well-founded legal assessments quickly in order to be able to act.

Those working in this field therefore need not only a solid understanding of the legal foundations but also the ability to respond dynamically to complex and rapidly changing situations in order to produce well-founded legal assessments. At the same time, international law continues to address key issues for the future, such as security, economic stability, and sustainability.