Iran has repeatedly threatened to close the Strait of Hormuz in response to any oil embargo or other unilateral sanctions taken against it. The Strait of Hormuz, depending on the reports you read, is at its narrowest somewhere between 17 and 30 nautical miles wide. The bordering States Oman and Iran both assert 12 nautical mile territorial seas. However, the deep water channels that are safe for tankers, used under an International Maritime Organization traffic separation scheme, are only two miles wide each. The outbound lane from the Persian Gulf passes through waters off Oman, the inbound lane through Iranian territorial waters. (Please correct me if I have any of this factual material wrong.)
What legal regime applies to the route through Iranian territorial waters? The ordinary starting point would be that a State may temporarily suspend innocent passage its territorial waters, without discrimination, for essential security reasons (Article 25(3), UN Convention on the Law of the Sea). However, as Hormuz is a strait used for international navigation, Iran lacks that ordinary power.
Under UNCLOS, where a strait is used for international navigation and there is no equally convenient route through open high seas waters, then “all ships and aircraft enjoy the right of transit passage, which shall not be impeded” (Art. 38(1)). This would seem decisively against Iran, but for the fact it is only a signatory to the UN Convention on the Law of the Sea and has never ratified it. The precise legal regime applying to Iran and the Strait of Hormuz is thus open to debate.
Some States, especially the US and UK, contend the UNCLOS regime of unimpeded transit passage is customary international law. The alternative is that outside UNCLOS there is only a customary international law right of non-suspendable innocent passage. The Corfu Channel case established in 1949 that warships, and a fortiori merchant ships, have a right of innocent passage through international straits which the coastal State may not suspend.
It was certainly held under the Corfu Channel case that in a time of heightened tensions Albania would have been entitled to regulate (though not prohibit or effectively nullify) the passage of warships through its waters. (See further the discussion in Churchill and Lowe.) Thus it is clearly arguable that under the non-suspendable innocent passage regime a coastal State retains its right to prevent non-innocent passage by individual foreign vessels; while under the UNCLOS transit passage regime it would lack any such rights of enforcement (though it would retain the right to formally regulate certain matters).
Thus, there is some basis for an argument that Iran could seek to restrictively regulate passage through its territorial sea short of suspending innocent passage – provided that as a matter of custom the Corfu Channel and not the UNCLOS rule applies.
However, in the comments to Sahib Singh’s recent post on Iranian sanctions Dan Joyner raised the question whether Iran could take countermeasures in the Strait in response to illegal interventions against its nuclear programme. Rather than close the Strait, Dan suggested Iran might be justified in seizing and confiscating vessels of the nationality of the States responsible for various illegal interventions against its nuclear programme (presuming these acts could be proven the responsibility of Israel and the United States).
Ordinarily, under the ILC Articles on State Responsibility, countermeasures must:
- be targeted only against the responsible State;
- be preceded by an offer to negotiate;
- consist only of the injured State withholding performance of one or more international obligations owed to the responsible State;
- be proportionate and readily reversible; and
- not involve the use of force.
Technically, seizing individual vessels under Dan’s scenario would not involve closing the Strait. Could it be described as suspending the right of innocent passage of certain targeted States? Perhaps, though I have some (possibly formalistic) qualms about the idea that suspending a freedom from interference can create a positive right to interfere. That aside, would seizing merchant vessels involve a prohibited use of force under the UN Charter? The majority view among scholars would appear to be that such a “police action” is not usually tantamount to a use of force (see e.g. Guyana v. Suriname), though much might depend on how such an interdiction operation was carried out.
The suggestion some vessels could be seized as a countermeasure is thus not implausible, but the real question would be sufficient proof of attribution of the complained of conduct to the targeted States.
Finally, one might note that actually closing the whole of the Strait by force could constitute a blockade of the ports of Kuwait, Qatar, Bahrain, the United Arab Emirates and Iraq. This would appear to be a prima facie act of aggression against these States as the General Assembly’s Definition of Aggression (UNGAR 3314) includes blockade of ports under Article 3(c). Such an act of aggression would, at a minimum, justify Security Council intervention though we could debate what other action might be permissible in such a case.
This is far from a fully developed analysis, so thoughts are welcome. My apologies if my replies to comments are less than timely.