A simmering dispute between the United States and Israel heated up this week as leaders of both countries traded sharp statements. On the surface, and according to most media accounts, the disagreement stems from Israel’s dissatisfaction that the United States has not articulated a red line that it will not allow Iran’s nuclear program to cross. But the Obama administration has articulated a red line. In fact, the U.S.–Israel spat is fundamentally about Israel’s dissatisfaction with where that line is drawn and its lack of confidence that the line can and will be enforced.

This week, Israeli prime minister Benjamin Netanyahu drew attention declaring that those “who refuse to put red lines before Iran don’t have a moral right to place a red light before Israel.” He was responding to Secretary of State Hillary Clinton’s statement from the day before regarding Iran: “We’re not setting deadlines.” Not surprisingly, this dispute has been interpreted as being about American willingness, or lack thereof, to establish a red line – a line beyond which Iran’s nuclear program will not be allowed to advance. But this is both a misreading of Clinton’s comment – deadlines are not red lines – and of Israeli concerns. At the heart of this dispute are actually esoteric questions at the intersection of foreign policy, intelligence gathering, military planning and nuclear engineering: what does Iran need to do in order to acquire nuclear weapons? What specific actions can prevent Iran from acquiring them? When will it be too late to prevent Iran from taking that last step? How much can we know about what Iran is actually doing?

There are two main schools of thought about where to draw the line on Iranian action. The first maintains that Iran cannot be allowed to acquire a nuclear weapon – meaning a fully-assembled, functioning nuclear device – but, by extension, anything short of that will be tolerated. The second argues that Iran should be stopped even before it is able to build a complete nuclear weapon – that it cannot be allowed to achieve even the capability to produce a nuclear weapon.

The distinction between these two approaches – nuclear weapon versus nuclear weapons capability – stems from the minutiae of nuclear technology. A nuclear weapon has three main components: first, fissile material either highly enriched uranium (HEU) or plutonium that can release massive, destructive amounts of energy; second, the device, or “weapon,” which creates the nuclear explosion by triggering a nuclear chain reaction in the fissile material; third, a delivery mechanism—bomb, missile, or some unconventional means—that delivers the weapon to its target.

A country can be considered to have developed a nuclear weapon once it has assembled the three main components of a nuclear weapon. Some might consider testing to prove weaponization, but the United States never tested the bomb it dropped on Hiroshima, partly because of a limited supply of fissile material. Nuclear weapons capability, on the other hand, is achieved when a country has all the requisite technology and components, but has not yet assembled them or tested a weapon.

The chief advantage of drawing the red line at a nuclear weapon is clarity. It is both clear what act would violate that red line and that such an act would also be illegitimate. Iran, as a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons, has committed not to build a nuclear weapon. The United States’ and its allies’ assertion that an Iranian nuclear weapon is unacceptable is clearly supported by international law.

Proponents of drawing the red line at capability believe that such clarity does not exist, and that ambiguity pervades both illicit nuclear programs and the international arena. This argument has three dimensions: technical, geopolitical, and intelligence. Technologically, fissile material production has historically been the most difficult and time-intensive hurdle to developing nuclear weapons. Other countries that have successfully built a nuclear weapon spent much more time producing fissile material than they did designing the actual weapon. For example, the bomb the United States dropped on Hiroshima was already on a boat in the Pacific while its fissile material was still being manufactured. Thus, making nuclear weapons capability effectively means a commitment to prevent Iran from producing highly enriched uranium.

Geopolitically, the negative consequences of Iran acquiring a nuclear weapon will begin well before it ever announces itself as a nuclear power.Indeed, because the costs of nuclear conflict are so grave, there is a strong incentive to err on the side of caution when estimating the nuclear capabilities of other countries. Thus, U.S. intelligence analysts came to the conclusion that North Korea either had a nuclear weapon or was capable of assembling one in the late 1990s – leading the military to adjust its planning accordingly. Still, North Korea did not publicly declare that it had a nuclear weapon until 2005 and not until 2006 did the reclusive dictatorship actually conduct a nuclear test. Similarly, as Iran’s stockpile of enriched uranium balloons and its centrifuges multiply, other countries would have no choice but to treat it as a nuclear power, becoming more cautious about provoking, and more willing to appease Iran, lest they spark a conflict that could escalate dangerously.

Finally, the entirety of Iran’s nuclear program is not fully transparent, limiting what we can know about it and when we can know it. International Atomic Energy Agency (IAEA) inspectors routinely monitor and report on Iran’s uranium enrichment activities, but those reports are based on access Iran allows to known facilities. IAEA reports provide credible data about how close Iran is to producing highly enriched uranium. But while the IAEA has strong evidence to suggest that Iran is also attempting to master the designs for a nuclear weapon, they have no way of verifying it.

Indeed, determining whether Iran is building a nuclear device would be exceedingly difficult. It would require precise intelligence gathering about when the small cadre of Iranian leaders responsible for the country’s nuclear program make the decision to assemble a weapon, and where that final turn of the screwdriver is occurring. United States officials have consistently maintained that they have the means to detect any such attempt to assemble a weapon. "We know generally what they're up to. And so we keep a close track on them,” Secretary of Defense Leon Panetta recently said. Adding that “we think we will have the opportunity once we know that they've made that decision, to take the action necessary to stop [Iran].” However, the track record of U.S. intelligence agencies in this field, as Israeli leaders are well aware, is not encouraging; they have never before predicted any country’s initial test of a nuclear weapon: not India’s, not Pakistan’s, not North Korea’s. This is not an indictment of the intelligence agencies but it does illustrate the difficulty in gathering this type of information – a difficulty that must be factored into any decision about setting red lines.

The debate between these two camps, as we have written previously, was already waged within the administration and Congress late last year and early this year, when there seemed to be some confusion about what the policy was or should be. In the end, the administration did resolve this internal debate and, despite some commentators thinking otherwise. It has drawn the line at, as President Obama has consistently and repeatedly said, preventing Iran from “acquiring a nuclear weapon,” even though a bipartisan group of 32 Senators urged him to draw it at capability. The administration’s decision gives the United States more time to attempt diplomacy and sanctions – avoiding setting a “deadline,” as Clinton said – but it has nevertheless been clear about its intentions if time ever does run out. As Panetta told CBS in an interview, “If . . . we get intelligence that they are proceeding with developing a nuclear weapon then we will take whatever steps necessary to stop it.”

The Israelis, on the other hand – partly out of concern for their own ability to strike Iran’s nuclear program – seem to have set their own red line at nuclear weapons capability and have become increasingly anxious as that capability draws closer. Indeed, based on the latest IAEA report released on August 30, Iran’s nuclear programis performing faster and better than in any prior period,despite tough sanctions and cyber warfare. Iran continues to produce 3.5 percent enriched uranium at the fastest rate ever – 62 percent faster than the end of 2011—and is using more centrifuges than ever before.Most disturbingly, Iran continues to accelerate its production of 20 percent enriched uranium; the production ratehas more than tripled since the end of last year.Iran’s stockpile of 3.5 percent and 20 percent enriched uranium could already yield, with further enrichment, enough highly enriched uranium (HEU), uranium enriched above 90 percent, for several nuclear devices. Our calculations, and those of some other experts, suggest that if Iran chose to produce a nuclear weapon, which it has yet to do, it might be able to produce 20 kilograms of HEU – the minimum needed for a crude nuclear device – in between 26 to 102 days, depending on the production process (there remains uncertainty about the technical capability of Iran’s centrifuges and the knowhow of its scientists). We estimate that that 26-102 day range could shrink by early 2013 to 8-88 days, if Iran continues on its current trajectory.

With IAEA inspections happening on average every 40-60 days, Iran theoretically could already produce enough fissile material for a nuclear weapon, move it to a secret site, and, potentially, install it in a weapon before the world ever found out. Using the upper bound of the range as a guide, Iran is not yet at nuclear weapons capability. But, given its current rate of production rate of 3.5 percent and 20 percent enriched uranium, it would reach that capability by early 2014. Once Iran is capable of producing enough HEU (uranium enriched above 90 percent) faster than IAEA inspectors can detect it,Israel’s ability to track down and destroy the elements of Iran’s nuclear program will diminish significantly. This, coupled with Iran’s moving more of its program into fortified underground facilities, is the “zone of immunity” that Israel has repeatedly warned it cannot risk allowing Iran to enter.

This trend clearly alarms Israel and leads some of its senior officials to argue for striking Iran’s nuclear facilities soon. Israel has conveyed that it would hold off if it had more confidence in the United States and was convinced that the United States, with our much greater firepower and ability to conduct a more sustained attack, would act if necessary before Iran becomes a nuclear power.

Fundamentally, the current tension reflects the difficult international dynamics at play over the issue of Iran’s nuclear program. The United States is attempting to not just prevent a nuclear Iran but also to stop an Israeli strike. Israel, on the other hand, is keeping an eye not just on Iran’s nuclear progress, but on the words and action of U.S. leaders. Rebuilding trust and cooperation between the two countries will prove critical to advancing their mutual interest in thwarting Iran’s nuclear ambitions.

Michael Makovsky, a Pentagon official during the George W. Bush administration, directs the Bipartisan Policy Center's Foreign Policy Project, including its Iran Initiative. Blaise Misztal is associate director of BPC’s Foreign Policy Project.

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