How Great a Concern?

Iranian threats to close the Strait of Hormuz

by William D. O'Neil

For thirty years and more, Iran has periodically threatened to punish its enemies by closing the Strait of Hormuz to tanker traffic, thus cutting off much of the world's oil supply and presumably driving prices to record heights. The Iranians actually attempted a partial closure (against Iraq and Arab states supporting it in its war with Iran) in 1987-88, but the effects came nowhere near what they had hoped – the attacks did relatively limited economic damage to the Arabs but prompted an American intervention that was costly to Iran.

Nevertheless, concerns about the threat persist. In a recent article, Caitlin Talmadge outlined a scenario under which, as she saw it, Iran could plausibly close the Strait for a month or more and do very serious economic damage. ["Closing Time: Assessing the Iranian Threat to the Strait of Hormuz," International Security 33, No. 1 (Summer 2008): 82-117.] This would be accomplished by a combination of extensive mine fields in combination with attacks by antiship missiles launched from sites ashore. A massive and very costly American effort would be necessary to re-open the Strait, she predicts.

In response, I published an analysis which questions some of Talmadge's key assumptions and assertions. ["Costs and Difficulties of Blocking the Strait of Hormuz," International Security 33, No. 3 (Winter 2008/2009): 190-5.] This is a brief summary of what I say there.


The first thing to bear in mind about closing the Strait is how much damage it would do to Iran and its interests even to try. A threat against the oil traffic is a threat against every oil user which would be bound gain Iran all but universal enmity. In such a situation Iran's enemies, and particularly the United States, would enjoy largely a free hand to act as they saw fit to break the blockade, and do what injury they chose to Iran in the process. Iran could find itself entirely stripped of air and naval defenses in the wake of U.S. strikes. The United States would probably find it necessary to seize many of the Iranian and Iranian-occupied islands in the Persian Gulf, and would scarcely be in any hurry to return them. The military bases and commercial facilities around Bandar Abbas would no doubt be extensively damaged and perhaps even occupied.

Moreover, the one state whose oil traffic would most assuredly be seriously affected is Iran. American naval forces effectively control the seas throughout the region and could block all ships carrying oil or oil products to or from Iran. Not only would this cut off most of the revenues of the Iranian government and most of the nation's foreign exchange, but Iran actually depends on imports for many critical petroleum products, due to its mismanagement of its own resources. Even the limited flow of oil through pipelines would not necessarily be safe from U.S. precision-weapons attacks.


The Strait of Hormuz and the waters of the Persian Gulf to the west of it are a good deal too broad and deep to permit them to be closed with a handful of mines, as often has been suggested – see the map of the area. In order to close traffic, it is necessary to mine all the waters lying between the 25-meter depth curves, a span of about 25 miles.

Talmadge suggests that Iran could accomplish this with a few hundred mines laid by a combination of submarines and surface craft over a period of about a week. This is little doubt that Iranian mining efforts could be a very serious nuisance, but there are factors which work to limit potential effectiveness a good deal, especially the following:


The antiship missile problem must be seen in an end-to-end perspective. The weak points in such systems have always been in their methods for finding targets in the first place and guiding the missile to a hit. Destroying or neutralizing these capabilities will quickly make antiship missiles ineffective, even if it proves difficult to destroy the missiles themselves (as it very well could).

Summing up

An attempt by Iran to close the Strait of Hormuz would involve tragic costs, but most of them would be paid by Iran. As long as the United States and its allies in the region remain alert to the threat and maintain adequate precautions there is little reason to fear catastrophic consequences.

Copyright © 2009, 2010 by William D. O'Neil

8 February 2009, updated 13 January 2010

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