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.