Such scenarios caused president Jimmy Carter to famously announce the Carter Doctrine on January 23, 1980, "Let our position be absolutely clear: an attempt by any outside force to gain control of the Persian Gulf region will be regarded as an assault on the vital interests of the United States of America, and such an assault will be repelled by any means necessary, including military force."
The conventional wisdom is that either country would do so by mining the strait, something that happened in the war between Iraq and Iran in the 1980s. The conventional American military wisdom has always been that if a country attempted to mine the strait it might cause some short-term disruption, but that US naval assets would clear it in relatively short order.
But a new analysis suggests that the reality is actually more complicated and less sanguine than conventional wisdom suggests. It finds, "The notion that Iran could truly blockade the strait is wrong - but so too is the notion that US operations in response to any Iranian action in the area would be short and simple."
The key question is whether Iran can harass shipping enough to prompt US intervention in defense of the sea lanes. According to Caitlin Talmadge, a political science doctoral candidate at the Massachusetts Institute of Technology, who writes in the current issue of International Security journal, the answer, especially since the United States has long pledged to keep the strait clear, is yes.
Talmadge notes that Iran possesses a larger stockpile of missiles and mines 10 times as powerful as those used in the tanker wars of the 1980s, the last period of sustained naval conflict in the gulf. Even if Iran managed to lay even a relatively small number of these mines the US certainly would act to clear the area. But, she writes, "The experience of past mine-warfare campaigns suggests that it could take many weeks, even months, to restore the full flow of commerce, and more time still for the oil markets to be convinced that stability had returned."
Projections based on past instances of US mine-clearing operations indicate that it could take a month or more to reopen the Strait of Hormuz if Iran were allowed to initiate even a small mine-laying campaign.
Even worse, if the US decided to clear the strait of mines, the potential for further military escalation would be high. In part, this is because United States' mine warfare assets are designed to be used only in non-threatening environments.
Thus, the US would want to locate and destroy any sources of Iranian attack on its mine countermeasure (MCM) ships. Specifically, it would want to eliminate Iran's land-based, anti-ship cruise missile batteries. The aerial hunt for these assets could add days, weeks, or even months to the time needed to clear the strait, and quickly develop into a large and sustained air and naval campaign.
Even if the strait were not closed during such a campaign, military conflict in the area could cause prices to skyrocket in anticipation of a supply disruption, and to remain high until markets could be assured that the flow of commerce had been restored.
Talmadge notes that when Iraq invaded Kuwait in 1990, temporarily halting the export of oil in both countries, the world price of oil more than doubled merely on the expectation of future shortages. Although excess global supply combined with increased Saudi production helped lower the price within a few months, it did not return to the pre-invasion level for nearly a year.
At a time when excess global capacity is lower and the price of oil is higher, blockage of the strait would pose a vastly greater threat to the world economy.
In the past, Iran showed restraint, even during the Iran-Iraq War, when considering whether to disrupt maritime passage in the strait. But if the United States or Israel attacked Iran, say, over its nuclear program, the restraint that previously characterized Iranian behavior in the strait might evaporate.
In terms of assets Iran is thought to be able to lay mines from any of its three frigates, two corvettes, and 10 fast missile boats. It also has three ships in the Persian Gulf that appear to have dedicated mine-laying capabilities, plus three still-functioning mine-laying helicopters. In addition, Iran possesses more than 200 smaller patrol and coastal combatants suitable for mine laying. Iran also has three relatively modern diesel-electric submarines from Russia, which each can carry 24 mines, although the Iranian operational record with submarines has been spotty.
Iran is believed to possess at least 2,000 mines. Although this is not a large stockpile by historical standards, even small numbers of mines have been able to halt surface traffic when their presence was known. Talmadge notes that in 1972 the US immediately stopped all traffic in and out of North Vietnam's Haiphong Harbor with an initial drop of only 36 acoustic-magnetic mines.
While the exact type of mines Iran has is unknown, it is likely that they have some beyond the classic contact type. Iran might have the sophisticated Russian-manufactured MDM-6, which can be delivered by ship or submarine. The MDM-6 detonates in response to acoustic, magnetic, or pressure influences within a radius of 50 to 60 meters.
Although it is impossible to predict exactly how many mines Iran could lay undetected Talmadge thinks it reasonable to assume Iran could lay several hundred mines in the gulf. She writes:
In terms of US efforts to clear mines, one can only extrapolate from the past. Traditionally, the goal in such operations is usually to clear an initial passage through a minefield in which the chance of hitting a mine is believed to be reduced to 10% or less and through which essential traffic can flow. The problem is that such a route might not be enough space for full traffic to resume or the oil markets to relax.
Also, the difficulty of clearing mines varies considerably with the type involved. Moored contact mines can be swept relatively easily once their location is known. Larger influence mines take more time and skill to identify and neutralize, often because the ships must carefully mimic the influences that will cause detonation.
And, as the US Navy plans to develop and deploy airborne mine countermeasures it is already phasing out its dedicated MCM ships.
Furthermore, such ships might be doing so in the midst of combat, unlike normal operating procedures. Even the most conservative estimates indicate that Iran probably possesses at least several hundred anti-ship cruise missiles. Iran would likely seek to fire them from shore in order to maximize their line of sight targeting and thus increase their range.
Even after taking into account topographical constraints about half of 33,000 square kilometers constitutes suitable terrain from which to target mobile missiles against traffic in the strait.
For the US to effectively counter this it would need a concept of operations for finding and destroying mobile missile batteries and targeting radars across approximately 16,500 square kilometers - an area about 50% larger than Kosovo.
How long would it take the US to destroy all the mobile batteries? Of course, estimates depend on assumptions such as the number of missile batteries Iran deploys, number of daily launches it conducts, and US's ability to locate and destroy them. Still, a best-case scenario, assuming a 100% success rate for the US in locating them, which never happens in real life, would take 18 days. In a less favorable scenario it could take 72 days to find all the batteries, assuming the US chose to keep searching, a dubious assumption.
In general, because US detection capabilities depend significantly on Iranian launches, Iran has some ability to stretch out the hunt.
Talmadge writes that Iran's limitations, such as the command and control and targeting challenges it would face in littoral warfare, are not often appreciated. But its strengths are often overlooked as well, such as the stocks of missiles and much more explosively powerful mines it has acquired since the tanker wars of the 1980s. She concludes:
Most important, Iran does not have to seal the strait entirely to provoke US intervention, and once that intervention begins, the potential for further military escalation is high. In particular, if the air and naval campaigns appear to be dragging on, the United States might be forced to consider holding hostage other targets in Iran or using ground forces. Either way, a significant and sustained increase in the price of oil would seem likely.