Hamed Jafarnejad/AFP/Getty Images Iranian Admiral Habibollah Sayyari says it would be "very easy" for his navy to shut down the Strait of Hormuz

Since it doesn’t have nuclear weapons yet, Iran is playing the lone trump card in its hand: threatening to shut down the Strait of Hormuz through which Persian Gulf oil flows to fuel much of the world’s economy. Iranian navy chief Admiral Habibollah Sayyari told state television Wednesday that it would be “very easy” for his forces to shut down the chokepoint. “Iran has comprehensive control over the strategic waterway,” he said as his vessels continued a 10-day exercise near the strait.

But just how good a trump card is it?

“Iran has constructed a navy with considerable asymmetric and other capabilities designed specifically to be used in an integrated way to conduct area denial operations in the Persian Gulf and SoH, and they routinely exercise these capabilities and issue statements of intent to use them,” Jonathan Schroden writes in a recent report for the Pentagon-funded Center for Naval Analyses. “This combination of capabilities and expressed intent does present a credible threat to international shipping in the Strait.”

Not so fast, other experts maintain. “We believe that we would be able to maintain the strait,” Marine General James Cartwright, then-vice chairman of the Joint Chiefs of Staff, told Congress last year. “But it would be a question of time and impact and the implications from a global standpoint on the flow of energy, et cetera, [that] would have ramifications probably beyond the military actions that would go on.”

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International maritime law guarantees unimpeded transit through straits, and any deliberate military disruption is an act of war. “Anyone who threatens to disrupt freedom of navigation in an international strait is clearly outside the community of nations,” the U.S. Navy’s 5th Fleet said from its headquarters in Bahrain. “Any disruption will not be tolerated.”

Of course, brandishing a threat and carrying it out are two different things. “By presuming that Iran can easily close the strait, Western diplomats concede leverage, and the current U.S. habit of reacting immediately and aggressively to Iranian provocations risks unnecessary escalation,” Eugene Gholz, a professor at the University of Texas, wrote in Foreign Policy in 2009. “Iran would find it so difficult, if not impossible, to close the strait that the world can afford to relax from its current hair-trigger alert.”

Most U.S. military thinkers, speaking privately, seem to agree. There are two linked issues at play here: military and monetary. While it might be challenging for the Iranian navy to shut down commerce flowing through the strait, Iranian moves to carry out that threat could have much the same effect. Oil companies, and the shippers that transport their product by water, are conservative business types, not given to putting their costly tankers and crews in harm’s way. But they’d get over it pretty quickly, and commerce would resume, with higher insurance rates.

One point worth noting: analyses of possible Iranian military action to plug the strait generally note that Iran gets about half of its national budget from oil exports that transit the strait. But if the next round of sanctions keeps Iranian oil off the world market, that brake on Iranian military action will be gone.

Iran has been practicing such saber-rattling for decades, and it always sends a nervous twitch through the world oil markets, spiking prices upward. It has done so this week, and oil’s per-barrel price has flirted with the $100 mark. That’s a drag on the world economic powers seeking to punish Iran for its nuclear-development efforts, and Tehran plainly views it as a net-positive for itself. That’s especially true in the year leading up to a U.S. presidential election, where the incumbent is seeking a second term.

About a fifth of the world’s oil flows through the strait, which is only 34 miles wide at its narrowest point. But the navigable part of the strait is 20 miles across, although shipping is supposed to use a pair of two-mile wide channels, one inbound and the other outbound. Iran borders the strait to the north and east, and it has a major naval base – and its key submarine base – close by.

“While closing the Strait may be possible for Iran for a short period of time, the U.S. military would prevail in a conflict with Iran in order to re-open the Strait at a great cost to the Iranian armed forces,” Brenna Schnars wrote in a 2010 study at the Naval Postgraduate School. “With international mistrust concerning the Iranian nuclear program already at the height of world concerns, an Iranian closure of the Strait would only enrage the majority of the international community, as their economies would severely suffer without its oil imports from the Persian Gulf.”

U.S. Navy Commander Rodney Mills examined the military implications of an Iranian move to shut the strait in a 2008 study at the Naval War College. His bottom line:

There is consensus among the analysts that the U.S. military would ultimately prevail over Iranian forces if Iran sought to close the strait. The various scenarios and assumptions used in the analyses produce a range of potential timelines for this action, from the optimistic assessment that the straits would be open in a few days to the more pessimistic assessment that it would take five weeks to three months to restore the full flow of maritime traffic.

But fighting an Iranian effort to close the strait may not be easy. Iran in recent years has acquired “thousands of sea mines, wake homing torpedoes, hundreds of advanced cruise missiles and possibly more than one thousand small Fast Attack Craft and Fast Inshore Attack Craft,” U.S. Navy Commander Daniel Dolan wrote in a report last year at the Naval War College. “…The majority of these A2/AD [anti-access, area-denial] forces are concentrated astride the vital Strait of Hormuz…” He urged the U.S. and its allies to fight any Iranian effort to shut the strait from the relative safely of the Arabian Sea, that broad body of water between the Persian Gulf and Indian Ocean. “It will allow the [allied commander] to concentrate fires on attriting the enemy forces,” he said, “while denying the enemy an equal opportunity to return fires.”

History offers some guidance. In the 1980s, the “tanker wars” between Iran and Iraq in the Persian Gulf – which led to 544 attacks and 400 civilians killed over eight years – the oil flow dropped by 25% before returning to normal levels. Insurances rates also would rise – perhaps from a penny to $6 a barrel, Mills estimates – a steep hike in insurance premiums, but not that much when tacked on to a $100 barrel of oil. “Despite the increased risk,” Mills notes, “history shows us that insurance will remain available at a reasonable rate for the value of the cargo shipped.”

Iran has scant chance of covertly mining the strait, U.S. military officers say. Small boats or anti-ship missiles would make more military sense. But Iran’s trio of Russian-built Kilo-class submarines, as well as a dozen smaller subs, would be vulnerable to U.S. anti-submarine warfare. “The (U.S.) Navy,” Mills wrote, “would be eager to permanently eliminate the Iranian submarine threat in a naval conflict.”

And attacks Iran launched against tankers aren’t guaranteed to work. “Most tankers today are of newer, double-hulled designs; coupled with internal compartmentalization, this tends to limit damage from an explosion,” Mills’ study said. “There are relatively few areas of vital machinery that could disable the vessel if damaged, and much of the vital machinery is underwater.” But what about all that oil? “The crude oil they carry tends to absorb and dissipate the shock caused by an explosion, reducing the effectiveness of the warhead,” Mills wrote. “And the crude oil is not very flammable, reducing the chance of fire or secondary explosion.”

All this is not to say any battle over the strait would be a cakewalk, as some U.S. officials erroneously predicted the Iraq war would be. If war were to break out, Iran would throw everything it has into the fight. “It’s clear that the Iranians have taken an approach in which they are going to attempt to use small boats, swarms, cruise missiles, mines, perhaps suicide boats, small submarines,” Vice Admiral Mark Fox, the top U.S. commander in the region, said earlier this year. “We watch them very carefully and understand where they are, what they’re doing.”

Fox’s 5th Fleet, which patrols the region, recommends its officers read Immortal: A Military History of Iran and Its Armed Forces, by CIA analyst Steven R. Ward. “Iran’s soldiers, from the famed `Immortals’ of ancient Persia to today’s Revolutionary Guard, have demonstrated through the centuries that they should not be underestimated,” a summary of the book on the fleet’s web site says. “The Iranians’ ability to impose high costs on their enemies by exploiting Iran’s imposing geography bear careful consideration today by potential opponents.”

Fox acknowledges that “imposing geography” cited by Ward as the admiral discussed how the Iranians would likely fight. “They have a long littoral there — it’s 1,300 nautical miles,” Fox said. “They’ve got a lot of places where they have an ability to set up, they have coves for small boats and cruise missiles that can potentially move around.” All this would complicate any conflict.

But Mills sees all the Iranian rhetoric and war gaming as little more than Persian saber rattling. “Iran gains more from the existence of their threat,” he concludes, “than they would by actually carrying it out.”