Although US Defense Secretary Leon Panetta and his Israeli counterpart, Ehud Barak, still disagree on the utility of continued sanctions, both now accept a more ominous understanding. The cumulative effect of sanctions designed to slow Iranian nuclearization has not worked thus far. Almost certainly, both acknowledge, and within a year, unless it is stopped at the 11th hour by substantially more immediate and tangible military measures, Tehran will enter the “Nuclear Club.”

What happens next? Whether attempted by the US or Israel, or both together, exercising a successful preemption option at this 11th hour would be numbingly problematic. At the same time, rejecting this military option altogether, on operational grounds, could expose Israel to intolerable conditions of extended vulnerability.

In essence, foregoing anticipatory self-defense, the legal equivalent of a permissible first-strike, could mean that Israel will have to live under the unending threat of a nuclear sword of Damocles. Even worse, unless it can be assumed that Iran’s leadership will remain consistently rational, thus always valuing national survival more highly than any other preference, or combination of preferences, there might remain few if any viable prospects for Israeli nuclear deterrence.

In such dire – but altogether conceivable circumstances – Israel’s only still-available foundation for physical survival might even be its active defenses.

What would this mean? The core of Israel’s active defense plan for Iran remains the Arrow anti-ballistic missile program. Iron Dome, a corollary and interrelated system, is intended primarily for intercepting shorter-range rocket attacks. At first glance, looked at exclusively from Israel’s technical side, everything may appear promising. In principle, at least, the implications of Israel’s no longer viable preemption option could remain tolerable.

Still, upon closer reflection, a grave problem emerges.

THIS DIFFICULTY lies in making untenable assumptions about any system of active defense. In short, in a world of multiple enemies, no system of ballistic missile defense, even the IAF’s newly upgraded interceptors known as “Block 4,” can ever be reliable enough to preclude an associated strategy of deterrence. Reliability of intercept is a “soft” concept. In reality, any missile-defense system will always have some “leakage.” Panetta and Barak are both well aware of this critical limitation.

A small number of Iranian missiles penetrating Arrow and related defenses might be “acceptable” if their warheads contained “only” conventional high explosives, or chemical high explosives. But if the incoming warheads were in any measure nuclear and/or biological, even an extremely low rate of leakage would be unacceptable.

In the next few months, Israel must move, recognizably, to strengthen its still-ambiguous nuclear-deterrence posture. To be dissuaded from launching an attack, a rational adversary would always need to calculate that Israel’s second-strike forces were sufficiently invulnerable to any contemplated first-strike aggressions.

Facing Israel’s Arrow, this adversary could require steadily increasing numbers of missiles in order to achieve an assuredly destructive first-strike capability. Still, once Iran was able to assemble a certain determinably larger number of deliverable nuclear warheads, Arrow could cease to serve the indispensable and lesser known deterrence function of its security mission.

ISRAEL MUST continue to develop, test and implement an interception capability to match the cumulative enemy threat. Simultaneously, it must also take considered steps to enhance the credibility of its still-opaque nuclear deterrent.

More precisely, if Iranian nuclearization proceeds unhindered, Israel should prepare to take its bomb out of the “basement” on very short notice, thus ending Jerusalem’s tacit nuclear policy of “deliberate ambiguity.” Israel, of course, has already operationalized a robust second-strike nuclear force. This force, hardened and dispersed, must now also be made more recognizably ready to inflict an unacceptable retaliatory salvo. As purely “counterforce” targeting could have inadequate deterrence benefits, Israel’s nuclear targets must be identifiable enemy cities. Soon, it may also be the optimal time to remind Iran of Israel’s expanding deployment of certain sea-based deterrent forces.

Always, Israel must clarify that Arrow and other defenses would operate simultaneously, or in tandem, with Israeli nuclear retaliations. Iran must be made to understand that Israel’s defensive deployments would never preclude, or render less probable, an Israeli nuclear reprisal.

In the best circumstances, Iran would never have been allowed to proceed toward full military nuclearization with impunity. Now, however, Israel will have to deal with a persistently recalcitrant enemy regime by implementing steady enhancements of its nuclear deterrence and active defense capabilities.

Although a regime-change in Tehran might first appear to be an attractive alternative option, any such transformation might offer Israel only temporary national-security benefits. Moreover, it is not out of the question that a successor regime in Tehran could prove even worse for Israel, and for the US.

Iranian nuclear harms could be directed toward Israel, not only via direct-missile strikes, but also by terrorist- proxy delivery systems, including cars, trucks and boats. Should a newly-nuclear Iran ever decide to share its weapons-usable materials and scientific personnel with Hezbollah in Lebanon, for example, Israel might also have to face a heightened prospect of nuclear terrorism. The considerable dangers posed could impact American cities as well.

Today, whatever their continuing disagreement on further sanctions, Panetta, Barak, and their respective heads of state, must make an unenviable operational decision. More than likely, in their prudent cost-benefit calculations, the preemption option will have to be rejected. Other plans will have to be made for dealing with a soon-to-be nuclear Iran.

Successfully deterring a possibly irrational nuclear adversary in Tehran is not out of the question.

After all, such an adversary might still have a consistent hierarchy of preferences. Directed by this sort of hierarchy, Iran’s decision-makers could reasonably calculate that the benefits of any long-term peace with Israel are actually greater than the expected benefits of war.

The task for Israel and the United States will then be to correctly identify: (1) Iran’s very highest preferences (almost certain to be religion-based); and (2) credible ways to most convincingly and substantially threaten these core Iranian values.

This task is formidable, but it can be accomplished.

Louis René Beres is Professor of Political Science and International Law at Purdue. General John T. Chain was commander-in-chief, US Strategic Air Command, and director, Joint Strategic Target Planning Staff.

Please LIKE our Facebook page - it makes us stronger