If Iran is going to pose a threat to Israel, it has to pose a credible one. The most catastrophic outcome for Iran's leaders is a nuclear attack that fails and leaves the nation a pariah, looking down both barrels of a justifiably enraged and very well-armed adversary.

Determining a nuclear threat's credibility involves weapons, delivery systems and defenses, both conventional and otherwise. The decision to use nuclear weapons against Japan rested on many factors, some so obvious now that we do not think about them: Japan no longer had any means to retaliate; qualms against the use of weapons of mass destruction had long been abandoned by the Allies with the use of heavy bomber raids; and a reliable delivery system was in place, to the point where a small flight of B-29s could raid Japan in daylight with a high probability of success and survival.

Iran's route to constructing such a force involves the parallel development of intermediate-range missiles and nuclear warheads. So far, no nation has succeeded in doing this either quickly or inexpensively. Conventionally armed, non-precision-guided missiles fired over regional distances are insufficiently accurate to do more than threaten random destruction and widespread disruption in large urban areas, while Iran is not within sight of any alternate delivery means for a nuclear device.

The rates at which the U.S. and Soviet Union produced both nuclear-capable missiles and warheads during the Cold War obscure the fact that both were costly and slow to produce in their early stages. They require a number of unique technologies (from high-temperature materials and liquid- or solid-rocket propulsion to nuclear material production and explosive devices). Engineering is complicated by the fact that a full-scale test necessarily destroys the device.

Iran has been assessed by some sources as having “hundreds” of Shahab-3 missiles—evolved from the Russian R-11/R-17 Scud via the North Korean Nodong—but fewer launch complexes. Although the missile is road-mobile on a semi-trailer towed by a Mercedes tractor, as a liquid-fueled weapon it has to be accompanied by fuel transporters and erected before refueling, rendering it an obvious, vulnerable target. (This is why the U.S. replaced the liquid-fueled Redstone by the solid-fueled Pershing after six years in service.) The solid-fuel Seijil—which requires a different industrial base to produce casings and propellants—is still in the testing stage.

Israeli researchers believe that Iran is working on disparate parts of a nuclear weapon system in parallel. “That's why they are conducting R&D work on the trigger mechanism and explosives behavior simultaneously,” Ephraim Asculai, senior research fellow at Tel Aviv University's Institute for National Security Studies, says. “Indications are that they did this work at [the nuclear research center at] Parchin. They had a working design. They had a metal vessel to contain the initiating and timing mechanism. The IAEA [International Atomic Energy Agency] asked to visit. The Iranians said, 'No.'

“They have the Shahab 3 and are working on solid propellants,” Asculai says. “And there is intelligence that they have been trying to fit a warhead to those missiles. They probably have several sets of warheads without the fissile materials.”

“Iran now has 5,000 kg (11,023 lb.) of low-enriched uranium,” he says. “But they only have 5,000 tons of raw uranium. That is not enough for a nuclear-power reactor, but it is more than enough for nuclear weapons. “They have enough enriched uranium to produce 3-5 weapons,” Asculai says. “The minimum arsenal is four—one for underground testing, a spare and two for a deterrent arsenal.”

“Later it will go very fast, but first they have to set up a different centrifuge cascade and some special containers,” Asculai says. “They will probably do it at the Fordow underground enrichment plant near Qom. This to my mind was the smoking gun when it was revealed in 2009. It has no civilian use. The Iranians are masters of concealment, so I think revealing the Fordow facility was an accident.”

Another detailed analysis by the Institute for Science and International Security (ISIS), issued in early October, concludes that Iran could create a “significant quantity” (SQ) of weapons-grade uranium (WGU)—25 kg, considered the minimum necessary for one device—in 2-4 months in a breakout scenario.

However, there is a catch. To do this, Iran has to deplete its existing stockpile of 2% low-enriched uranium (LEU)—with the result that subsequent SQs take longer, because the process has to start from scratch, with non-enriched material.

ISIS estimates Iran could produce two SQs in 4.6-8.3 months—based on computer simulations of the centrifuge installations at the Natanz plant—and could, with changes to the enrichment sequence, produce up to four SQs in 8.9-12.8 months. But that would deplete LEU stocks to the point where each additional bomb-load of material would take 9.5-17 months at Natanz. The smaller Fordow plant would take at least 21 months to produce a single bomb, according to ISIS.

Iran can put itself in a stronger position to attempt a breakout only by delaying the attempt and concentrating on bulking up its LEU stockpile—but this would mean losing ground against Israeli's improving defenses. And Iran remains burdened by international sanctions.

Moreover, continued Iranian protests about “sabotage”—including allegations that IAEA inspectors are directly involved—suggest that undercover attacks on the nuclear program, including recent alleged bomb attacks on power supplies, are still not under control, following the success of cyberattacks. The ISIS modeling does not take such problems into account.

Neither does 25 kg of WGU equal a bomb, let alone a missile warhead. The bomb itself has to be engineered and tested. And so far, only France, with practical experience of nuclear-weapons design, has declared a warhead operational without a physical test. Finally, the device must be integrated into a reentry vehicle (RV) that protects and powers it and fuzes it reliably at the correct altitude.

Iran also has to determine what constitutes a credible threat in the face of Israeli defenses—in a situation where the balance of attack and defense is the reverse of what prevailed in the Cold War. Unless and until Iran can develop effective penetration aids, such as maneuvering or low-radar-cross-section RVs and decoys (and that is likely to be a long time, since these constituted some of the most difficult projects of the Cold War era), the only available tactic to reduce the effectiveness of the defenses is a costly salvo attack.

Israel, meanwhile, is upgrading its defenses to deal with exactly that threat, including two entirely new interceptors and a major upgrade to the existing Arrow 2 missile. The Arrow System Improvement Program represents the latest evolution of the weapon, capable of intercepting faster targets at longer ranges.

The next step is the Arrow 3 exo-atmospheric missile interceptor, currently in development. The program was slated for the first test flight this month but that test has been postponed. With a thrust-vectoring kill vehicle designed for hit-to-kill intercept, Arrow 3 will provide the upper tier for the Israeli missile defense system, engaging hostile missiles in space, through their midcourse phase.

The Arrow 3 has twice the range, but only half the weight, of the Arrow 2 missile. The Arrow 3 receives a continuous stream of targeting data from optical sensors and the second-stage terminal booster has its own motor, giving it enhanced maneuverability.

An Arrow 3 battery alone is expected to be able to intercept a five-missile salvo within 30 sec., including the ability to launch missiles that cover multiple targets: The missile is agile enough to be redirected onto a different target after launch.

Another significant change will take place in 2013, as the new David's Sling missile system, currently in final developmental testing at Rafael, will reach initial operational capability. Unlike the task-specific Arrow 2 and Iron Dome, David's Sling was developed as a flexible, multi-purpose weapon system capable of engaging aircraft, cruise and guided missiles, and long-range ballistic rockets. Designed for land-based, maritime and airborne applications, the weapon—based on a common missile known as Stunner—is fitted with a dual-band seeker (radar and imaging infrared) and a powerful multi-pulse rocket motor that increases endgame maneuverability at extended ranges.

Israel's missile defense forces could also revive the shelved Missile Optimized Anti-Ballistic project, proposed during the 1990s. The concept was to shoot down hostile missiles during their ascent phase, using short-range Python missiles launched from high-altitude, long-endurance UAVs. However, at the time there were no suitable long-range vehicles available. Today, the Heron TP HALE UAV and the Stunner would be able to revive the concept. Along with intercepting missiles, a UAV would be able to provide early warning and tracking to the defense system.

Another key to Israel's defensive capability is the revolutionary decision-making technology produced by Rafael partner mPrest—and first deployed in the Iron Dome short-range rocket defense system—to tie together the missiles and new sensors and manage complex engagements.

Finally, Israel is working closely with the U.S. on missile defense, including exercises and simulations showing that, in a crisis, its defenses could be reinforced by land- or sea-based U.S. assets.

Iranian planners also have to worry about other countermeasures. Until a quick-reaction solid-fuel missile can be deployed, missiles are vulnerable on the ground. Mobile weapons are susceptible to electronic attacks on their command-and-control links—not simply jamming, but also cyberattacks that could covertly interfere with the missile and launcher control systems.

The Israeli developments present Iranian planners with a dilemma. Clearly, a number of missiles are needed to pose a credible threat of “unacceptable damage” that would lead at least to a nuclear stalemate with Israel. However, that number will rise considerably over the next few years, as David's Sling and Arrow 3 are deployed—by an extent that Iran can only estimate, based on its best intelligence. That could make a rapid breakout tempting, but if the ISIS report is correct, a rapid breakout produces only a handful of warheads followed by a long gap—certainly long enough for Israel to produce its next round of improvements.

The result is that Iran's nuclear weapons program is engaged in a race with Israel's defenses—a race that Israel started in the 1980s. If Iran attempts a breakout—leading to testing a nuclear device as soon as possible—it may actually hamper its ability to build a nuclear arsenal that it can use without the potential of a catastrophic failure. At the same time, it risks provoking (indeed, justifying) a physical attack that would set its program back even more. And with its economy already suffering due to sanctions, Iran has no more resources to devote to further military expansion.