Adm. Jonathan Greenert made an important observation last fall from the tower of the aircraft carrier USS John C. Stennis while in the Strait of Hormuz on the southern coast of Iran, the world´s busiest oil-shipping lane.

The chief of naval operations was sailing in a flotilla that showed off the Navy´s overwhelming power to strike at long distances: F-18 fighter jets, Tomahawk cruise missiles and deck guns able to fire a shell 15 miles.

Yet in the claustrophobic waters of the strait, which narrows to just 24 miles, Adm. Greenert noted that all that long-range firepower could potentially be countered by the Iranian patrol boats that came out to track the U.S. warships. Faced with a fight in close quarters, Adm. Greenert told a Senate panel recently, "You also may need a sawed-off shotgun."

As the U.S. and other Western powers prepare to meet Saturday in Istanbul with Iran to resume negotiations over its nuclear program, the U.S. military is sharpening its contingency planning. Advocates of peaceful engagement say economic sanctions against the Islamic regime are starting to bite, and are hopeful that Tehran will give up its uranium-enrichment program. Iran says the program is for use in electricity generation, but intelligence services say the regime is close to developing the capability of building a nuclear weapon. The Obama administration plays down the chances of a breakthrough at this meeting, the first face-to-face encounter between US. and Iranian diplomats in more than a year, saying the best outcome may be agreement for a second round.

Should all else fail and the U.S. or Israel decide to attack Iran, say analysts, they would face a miniature version of the U.S. military, circa 1975—sustained, barely, by a world-wide spare-parts bazaar. Experts say the Islamic Republic´s claims of advanced weaponry—such as armed, Predator-style drones—are mere boasts.

Military officers and defense analysts say the U.S. could quickly overwhelm Iran´s air defenses, leaving evenly spaced bomb craters, for example, on runways to disable Iranian air bases. Pinpoint airstrikes would attempt to destroy all Iran´s known nuclear facilities—a goal complicated by the fact that the regime has buried some of its production sites. The Pentagon is rushing to upgrade its largest conventional bomb to better penetrate fortified underground facilities.

Naval officers believe Iran would retaliate by waging the naval equivalent of guerrilla warfare in the Persian Gulf by mining the Strait of Hormuz or swarming U.S. naval vessels with small boats.

Such threats, so-called asymmetric warfare, could prove as dangerous and unpredictable as roadside bombs in Afghanistan or Iraq, with an low-cost mine potentially crippling or sinking a billion-dollar warship.

In such a scenario, the U.S. military would face a time-consuming and often perilous effort to reopen shipping lanes to international oil traffic.

"They have stayed true to their stripes," said a senior military officer in the Middle East. "They have always taken an asymmetric approach, going back to the ´80s."

Before the 1979 Islamic Revolution, Iran had among the most formidable conventional arsenals in the region, equipped with modern weaponry sold to the Shah by U.S. defense firms.

Iran´s military was later battered during eight years of war with Iraq in the 1980s. Iran has since cobbled together an array of weapons—some homegrown but much acquired from China, North Korea and the former Soviet Union.

Iran has already threatened to block the Strait of Hormuz in response to tighter international sanctions. Military analysts now estimate Iran has amassed as many as 5,000 naval mines, ranging from rudimentary devices that explode on contact, to high-tech mines that, tethered to the sea floor, can identify the acoustic signature of specific types of ships and explode only under the richest targets.

Scott Truver, a mine warfare analyst, said finding and clearing Iranian mines would be a cat-and-mouse game for the Navy. Mine warfare, he said "is as tough and dangerous as the IEDs on land were. Mines are equally hard to detect, if not harder."

The U.S. Navy knows firsthand. In April 1988, the frigate USS Samuel B. Roberts struck an Iranian mine, which blew a hole the size of a pickup truck in the hull, and nearly sank the ship. The U.S. retaliated by attacking two Iranian oil platforms and sinking several Iranian vessels.

Among the newest threats are sophisticated torpedoes Iran acquired from Russia that can home in on the turbulence of a ship´s wake and aren´t easily fooled by the decoys commonly used by warships.

Military planners worry about torpedoes launched from Iran´s three Russian-built Kilo submarines, as well as approximately four North Korean Yono-class mini-submarines, the class of vessel that sank a South Korean warship in 2010, killing 46 sailors.

Iran´s mini-subs cannot range far or stay long under water. But in the close quarters of the Strait of Hormuz, they could be easily positioned for attacks.

Iran also is known for its fleet of hundreds of small speedboats that can carry everything from machine guns to large antiship missiles. While a single speedboat may not imperil a warship, a swarm of small boats could overwhelm a larger ship´s defenses. In early 2008, a cluster of Iranian patrol boats sailed close to a convoy of U.S. warships. No shots were fired, but the provocation underscored potential dangers.

Conventional naval vessels aren´t the only concern. Iran can deploy mines or even missiles from merchant vessels, or dhows. Such threats would be nearly impossible to spot in the crowded shipping lanes of the Persian Gulf.

Ten years ago, the Rumsfeld-era Pentagon held a top-secret war game to test a Persian Gulf scenario. A maverick Marine Corps general, Lt. Gen. Paul Van Riper, led the "Red Team," the fictional Iranian adversary. Gen. Van Riper relayed orders to his front-line troops by motorcycle messenger, so the U.S. could not hack into his networks; he sent out speedboats armed with missiles and explosives to swarm U.S. warships. After the fictional smoke cleared, more than a dozen U.S. warships were at the bottom of the Persian Gulf.

That exercise, known as Millennium Challenge, was a wake-up call about the potential of asymmetric warfare. The Navy has since unveiled plans to boost the defenses of its ships in the Gulf.

Adm. Greenert said the Navy is interested in new robotic underwater vehicles that can search for mines and submarines and improved Gatling guns to counter Iranian small-boat attacks. The Navy has rushed to test and field a new anti-torpedo torpedo—a weapon that would potentially counter Iran´s more sophisticated torpedoes.

The Navy recently announced plans to double its fleet of Avenger- class minesweeping ships in the Persian Gulf.

The U.S. military is taking other steps. Earlier this year, the Pentagon unveiled plans to refit a transport ship as a staging platform for different kinds of missions, from countering mines to launching remotely piloted aircraft. It also could be used as a platform for launching commando operations with small patrol boats to intercept Iranian vessels, escort ships or protect oil platforms.

Beyond the waters of the Persian Gulf, military planners worry about Iran´s expanding arsenal of ballistic missiles, built with North Korean cooperation and know-how. The Defense Department estimates Iran has around 1,000 short- and long-range missiles that can travel from 90 to 1,200 miles, the largest inventory in the Middle East.

The longer-range Shahab-3, which could reach Israel, has received the most attention. But Iran´s shorter-range Scuds are on mobile platforms, allowing them to more easily evade detection.

Within striking distance of Iranian missiles are U.S. Army installations in Kuwait, a command post in Qatar, and the U.S. Fifth Fleet in Bahrain.

While relatively inaccurate, those missiles may have the potential to strike panic or provoke a wider war if they hit U.S. allies in the region. A retired Navy officer said the missiles don´t have sophisticated targeting but could score a blind hit on a Saudi oil field, a Qatari gas production facility or a city in the United Arab Emirates. "Face it, how accurate does it need to be?" he said.

Officials with Iran´s elite Revolutionary Guards threaten reprisals against any country used as a launch pad for strikes against Iran. A conflict with Iran, then, could be a real-world test for U.S. missile- defense plans. As part of a shift from Bush-era missile defense, which focused on defending U.S. territory from a long-range missile attack, the Obama administration has sought defenses against shorter- range Iranian missiles targeting U.S. troops overseas, as well as allies.

There is also a presumed terror threat. Iran´s Ministry of Intelligence and Security could activate so-called sleeper agents for acts of sabotage or terror attacks, according to U.S. officials. Militants sponsored or trained by Iran might attack U.S. diplomatic facilities in Iraq or bases in the Middle East.

"The assumption is that there are sleeper cells all around that would be activated in some way," said retired Marine Corps Gen. Anthony Zinni, the former head of U.S. Central Command, the U.S. military headquarters that oversees the region.

Military professionals generally agree that U.S. forces would quickly overwhelm Iran´s air defenses. Former Air Force Chief of Staff Gen. T. Michael Moseley, an architect of the shock-and-awe air campaign against Saddam Hussein in 2003, said a U.S. air campaign could inflict "a sense of strategic paralysis" on Iran´s air defenses by targeting command-and-control facilities, early warning radars and airfields.

But, Gen. Moseley said, Iran´s air-defense system—comprised of mostly older U.S. Hawk missiles and some surface-to-air missiles of Soviet design—was "not a trivial" threat to U.S. aircraft. "Anything that shoots at you merits some respect," he said.

Military officials said Iran´s forces shouldn´t be entirely discounted. In the late 1970s, the Iranians "had all the latest and greatest stuff" from the U.S., said Richard Brown, a Navy fighter pilot who helped train Iranian aviators in Isfahan.

Iran maintains a fleet of Vietnam-era F-4 and F-5 jets, according to defense analysts; its helicopter fleet, which includes versions of the Chinook, the Cobra and the Huey, would look familiar to a U.S. military veteran.

It still flies the F-14 Tomcat, made popular in the movie "Top Gun." Iran was the only foreign military customer for the F-14, once a high- end U.S. fighter.

Today, many of these aircraft are close to the end of their service life. Aviation experts say Iran keeps them airworthy by cannibalizing and reverse-engineering spare parts. Iran bought nearly 80 of the F- 14s. Analysts believe around 25 can still fly. By comparison, Saudi Arabia´s fleet of U.S.-made F-15 fighters outnumbers Iran´s F-14s by about six to one.

Veterans of the 1970s training programs in Iran doubt the Iranians have maintained enough parts to keep its U.S.-made aircraft in flying condition. Ric Morrow, a naval aviator who worked on the Iranian F-14 training program, said what remained of the Iranian air force would be "no contest" for the U.S.

The air-to-air weapons built for Iran´s aircraft also may have outlived their shelf life. Steve Zaloga, a missile expert at the Teal Group, a defense consultancy, said the solid rocket motors and batteries go bad over time.

Some evidence suggests, however, that Iran operates a global procurement network to buy spare U.S. military parts. Since 2007, the U.S. Justice Department has handled more than two dozen export and embargo-related criminal prosecutions related to military spare parts destined for Iran.

Clif Burns, an export attorney at the law firm Bryan Cave in Washington, D.C., tracks such cases. He said Iran appeared to give shopping lists to independent contractors who buy parts in the world´s aviation market. "The procurement effort is pretty large and enforcement alone isn´t able to stop the flow of aircraft parts into Iran," he said.

—Jay Solomon contributed to this article. (Copyright © Dow Jones & Company, Inc.) 04/14/12)