In response to international economic and diplomatic pressure to halt its nuclear program, Iran is reportedly contemplating closing the Strait of Hormuz, a narrow and critically important waterway through which approximately a third of global sea-based oil shipments pass each year. The precise nature of this potential action is a little unclear from media reports. Some accounts state that the closure would pertain only to foreign warships that do not receive Iranian permission to transit. Others give the impression that Iran may bar all transit, including oil shipments. The difference is significant, but many seem to think that Iran would be acting illegally either way. My aim here is to briefly explore that view under international law.

The principal hurdle to either type of closure is the U.N. Convention on the Law of the Sea, a treaty that Iran has not ratified but that is widely accepted as codifying preexisting customary rules that bind parties and non-parties alike. One such rule is that in a strait all ships and aircraft shall enjoy an unimpeded right of “transit passage,” which is “the exercise . . . of the freedom of navigation and overflight solely for the purpose of continuous and expeditious transit of the strait” (art. 38). A corollary is that states bordering straits “shall not hamper transit passage,” and that “[t]here shall be no suspension of transit passage” (art. 44).

Closure of the Strait to all transit would clearly violate these rules. The more interesting question is whether closure only with respect to unauthorized foreign warships would also be illegal. To justify this action, Iran might argue that although the right of “transit passage” must be unimpeded, foreign warships cannot claim to exercise the right when their presence in the Strait is at least in part for a military purpose, and that refusal to grant authorization to transit in such circumstances is therefore perfectly permissible. The U.S. Navy, for example, might enter the Strait in part to contain Iran, fight piracy, signal a commitment to regional stability, or accomplish some other military objective. In those cases, the Navy would not be in the Strait solely for the purpose of “continuous and expeditious transit,” and therefore not be engaged in transit passage, and therefore lack the right of unimpeded passage under the treaty articles mentioned above, and Iran could permissibly decline authorization for the Navy’s vessel to pass through.

Such an argument has at least two significant problems, however. One is that while Iran’s territorial waters extend up to 12 nautical miles from its shoreline, the Strait is 21 nautical miles wide at its narrowest. Thus, to close the Strait, Iran would have to prohibit transit beyond its territorial waters, over at least 9 miles of ocean extending from the limit of those waters to the other side of the Strait. Nothing in the Law of the Sea Convention gives Iran a general power to do so.

The second problem with the argument is that, as a factual matter, the intent of the potential Iranian action appears to be to block the U.S. Navy from entering the Strait regardless of purpose. If executed in accordance with this intent, the action would violate the U.S. Navy’s right of transit passage in any case where U.S. warships aimed to use the Strait solely for the purpose of traveling to another destination.