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Iran’s oft-sounded threats to close the Straits of Hormuz to international shipping and thereby stop the flow of Gulf oil seem to have increased in frequency and intensity, apparently in light of the decision to impose harsher sanctions against Iran. Senior Iranian officials have thus warned that “any act of aggression or adventure” – in practice this means inspections of cargoes of Iranian ships (a step included in the Security Council resolution on Iran) – would generate an “appropriate” Iranian response and turn the Straits into an unsafe place for Iran’s enemies.

In general, these pronouncements are designed to deter the international community from fulfilling its resolutions and raise the cost of any military confrontation with Iran. The former commander of the Revolutionary Guards naval force, Rear Admiral Morteza Safari, warned that “American warships are easy prey to the Iranian navy,” and that for the “over 100 foreign naval vessels” currently in the Gulf, Iran would sail “100 vessels of its own.” Despite the fact that similar threats have never been realized in the past, it is important to examine possible scenarios, as the Straits are a choke point of critical significance for both regional and global stability.

The Straits of Hormuz are considered one of the most important naval passages in the world, and any interference to the flow of oil through them would immediately affect the global energy market. An overwhelming majority of the Gulf’s oil exports – some 90 percent – go through the Straits. The American Energy Department estimates that some 17 million barrels of oil – 20-30 percent of global consumption – pass through the Straits daily, in addition to 20 percent of the world liquid natural gas transports. Every day, some 15 tankers make their way through the Straits, most of which are headed for the Asian markets (e.g., over 75 percent of Japan’s energy consumption comes from the Gulf).

The oil moves in tankers through the Straits, whose narrowest point is 33 km. The tankers move into the Gulf in a 3.2 km-wide shipping lane, with a lane of equal width for outgoing traffic (a 3.2 km wide no-traffic lane separates the two active ones), so at no point does the total width of the international crossing exceed 10 km. The international shipping lane at the entrance to the Gulf is in Oman’s territorial waters but farther up the tankers enter an area that Iran claims as within its sovereignty. Accordingly, it would be relatively easy to interrupt naval transportation to and from the Gulf.

Even in the unlikely possibility that Iran could seal the Straits hermetically over a long period of time, such a move is not in Iran’s own best interests, as it would interfere with the import of refined oil to Iran and Iran’s export of crude oil (representing some 80 percent of its income) and would almost certainly lead to a confrontation with the American navy. Therefore, Iran has never attempted to block the Straits, certainly not fully. Operationally, such a move would require extensive naval mining in the Straits, something Iran found difficult to accomplish clandestinely in the past and which is tantamount to an act of war. At the end of the Iran-Iraq War, Iranian attacks on naval vessels generated an American response, whereby American navy ships escorted Kuwaiti oil tankers to and from the Gulf. In another case, after an American frigate hit an Iranian naval mine, the United States launched Operation Praying Mantis, during which the American navy sank most of Iran’s usable naval forces in the Gulf, putting them permanently out of commission.

Because of the Iranian navy’s weakness and America’s military superiority in the Gulf, Iran has placed priority on acquiring and building a large number of small, fast-moving vessels (some of which are for unmanned use) and has re-outfitted civilian vessels for military missions. As a result, in recent years there have been reports of Revolutionary Guard naval vessels skirmishing with American vessels. In January 2008, Iranian boats approached American vessels in a threatening manner, using the so-called swarm tactic: many small, fast-moving boats, some laden with explosives, intend to attack the target simultaneously from different directions (together with support from air and land fire) in order to stun the enemy’s defensive systems and limit the enemy’s capacity for responding effectively.

Recent assessments indicated that the American 5^th Fleet is capable of opening the Straits to naval vessels within a week. Such assessments may be based on the fundamental weakness of the Iranian air force, a belief in the American capability of paralyzing Iranian positions near the Straits, the US' improved ability to remove naval mines, and the fact that the Straits are deep and wide enough not to allow them to be easily blocked. In addition, unlike other vessels such as cargo ships, tankers are hard to sink due to their size, structure, and the nature of crude oil. Nonetheless, senior American military sources express themselves with caution: "Iran is developing its conventional military with 'limited' offensive missiles and naval assets able to disrupt Gulf shipping," and it "has the ability to restrict access to the Straits of Hormuz with its naval forces temporarily and threaten U.S. forces with missiles."[1]

Moreover, new Iranian means and improved tactics may present a more significant challenge to the American navy. As early as 2008, Iran already announced that it was building new bases, which could threaten movement through the Straits. The Iranians may also have introduced new types of vessels into service, such as mini-subs and improved weapons based on asymmetrical tactics.

Any American attempt to open the Straits would require at least partial removal of naval mine fields and dealing with the Iranian shore-to-sea missiles. Moreover, since such a confrontation is liable to develop into a more widespread campaign, questions arise regarding the West’s ability to limit the confrontation in time and space.For example, it may be that in tandem with an Iranian blockade of the Straits there would be an attack on the western shore of the Gulf, where there are strategic infrastructure facilities such as ports, refineries, and desalination plants.

Even in a more optimistic scenario (a partial blockade of the Straits with rapid, effective international action to open them), the impact of a “limited” campaign on the global energy market is liable to last a long time, beyond the event itself, because of the concern about a lack of supply. A longer event might necessitate the use of strategic reserves, taking advantage of the redundancy of global manufacturing capabilities (greatly limited because it is primarily concentrated in the Gulf region) and using alternate shipping routes such as the Saudi pipeline moving oil from the Gulf to the Red Sea (5 million barrels per day) and the Habshan-Fujairah pipeline, which is supposed to start operating next year and ferry 1.5 million barrels per day from the Gulf daily.

The Iranians apparently prefer to focus on harassment, i.e., pinpoint systematic attacks while attempting to cover up their responsibility for the events (e.g., by using civilian platforms or by acting outside of their own territorial waters). Because of its fundamental military weakness, Iran is incapable of blocking the Straits completely for long and will therefore focus on disrupting the freedom of movement in the Gulf in general, while attempting to avoid a comprehensive campaign that might cost it dearly – militarily, politically and economically. Until then, it will continue to threaten to close the Straits, a move that serves it well even if it is contrary to its own basic interests, while using the Straits’ unique geographical conditions and global sensitivity to tremors in the world’s energy market.

[1] .Viola Gienger, "Iran Could Get Bomb Uranium, Block Oil, Pentagon Says," Bloomberg, April 14, 2010.