Most Western audiences would not have regarded such a defense as entirely complete or truthful. But an explanation along those lines—that Iran had seriously considered but ultimately rejected nuclear weapons—could at least have been a face-saving way for Iran to acknowledge past activities and cooperate with the IAEA’s investigation, while at the same time continuing to deny that it had ever taken a decision to acquire nuclear arms.

Iran’s leaders decided not to go that route. Instead, they opted for the Big Lie: that Iran was never interested in nuclear weapons and that evidence of past nuclear weapons–related activities was fabricated as part of a conspiracy to discredit, sanction and ultimately bring down the Islamic Republic. It is a tribute to Tehran’s skillful public diplomacy and propaganda efforts, especially after the departure of President Ahmadinejad and arrival of President Rouhani and his team, that much of the world no longer pays attention to the IAEA’s November 2011 report and has forgotten Iran’s well-documented record of cheating on its nonproliferation obligations. To many countries, especially among the non-aligned group, Iran has successfully portrayed itself as the aggrieved party—a victim of false accusations who has had to endure a manufactured crisis.

In the wake of the IAEA PMD report, the Board resolution and “implementation day,” a number of observers will be disturbed that Iran appears to be getting away with the Big Lie, benefiting from sanctions relief while stonewalling the international community on its past nuclear weapons–related work. We can expect objections to giving away the sanctions leverage that provides the only plausible means of getting Iran to address past activities seriously. Some critics may call for withholding sanctions relief, or imposing new sanctions, until Iran makes a credible disclosure about the past.

So the likely outcome of the PMD issue will be untidy, to say the least. It will leave many past issues unresolved. It will fall short of how many observers expected the matter to be concluded. It will result in renewed criticism of the nuclear deal.

Is an Iranian confession necessary?

Should this unsatisfying PMD outcome affect our basic evaluation of the deal? In particular, will Iran's failure to disclose its past activities be a serious impediment to gaining confidence that Tehran is not pursuing nuclear weapons–related work in the future?

These questions will be hotly debated. But given the range of tools at our disposal for deterring and detecting Iranian non-compliance, Iran's unwillingness to come clean about the past does not represent a critical shortcoming.

The JCPOA provides significant tools to prevent future weaponization activities. It goes beyond the Nonproliferation Treaty by explicitly prohibiting specific activities related to nuclear weapons development, such as nuclear explosive modeling and research on explosive detonation and neutron initiation systems. In addition, the consensus procedure for the JCPOA's procurement channel, together with Security Council restrictions on importing sensitive technologies, will impede possible efforts to acquire materials and equipment relevant to weaponization.

Deterring and detecting weaponization, however, will not depend entirely or even primarily on the JCPOA. National intelligence capabilities, mainly those of the United States and its foreign partners, will play a critical role. Indeed, such intelligence capabilities provided crucial information about Iran's previous nuclear weapons work.

It is sometimes argued that full Iranian disclosure is essential to designing an effective JCPOA monitoring system. But the provisions of an agreement that could be most effective in monitoring small-scale weaponization activities would be more intrusive than any sovereign state would be willing to accept (e.g., keeping close track of all scientists with the necessary expertise, on-site verification of all equipment in the country that could be used in nuclear weapons design and diagnostics). With or without full knowledge of past Iranian activities, it would have been nearly impossible to reach agreement on such intrusive arrangements.

That is why deterring weaponization must depend less on agreed monitoring measures and more on national intelligence capabilities, procurement restrictions and the threat of strong penalties for non-compliance. Moreover, the production of nuclear weapons–usable material is the critical pacing factor in any nuclear weapons program, not experience with weaponization; the JCPOA provides reliable means of detecting the production of such nuclear materials.

Another argument for insisting on full disclosure is that it would indicate how much progress Iran had made on weaponization and that such information would be crucial in calculating how much time would be available to intervene to stop any future Iranian attempt to “break out” and build nuclear weapons. But from its own sources, the United States already has considerable knowledge of past Iranian nuclear weapons work. And in any event, in calculating how much time it would have to thwart an Iranian breakout, the United States would have to make the conservative assumption that Iran had made substantial headway in weaponization and would not require much time to proceed from the production of fissile material to the fabrication of a weapon. It is unlikely that anything the Iranians might say about past weaponization efforts would affect U.S. planning to stop an Iranian breakout, especially because whatever they said would hardly be taken at face value.

The strongest argument for insisting on full Iranian disclosure is that the admission of a previous intention to develop nuclear weapons would provide a measure of confidence that Iran had genuinely made a strategic decision to abandon the ambition of becoming a nuclear-armed state. But such a confession, which would contradict Tehran's core narrative that its nuclear program has always been peaceful as well as the Supreme Leader's fatwa that Islam forbids nuclear weapons, was never in the cards.

While a truthful confession would be valuable, it is not necessary for a sound agreement. More important than hearing Iran's version of the past is having confidence that nuclear weapons will not be pursued in the future. With rigorous implementation of the JCPOA's unprecedented monitoring measures, strict enforcement of compliance, enhanced intelligence gathering and a credible threat of forceful intervention to thwart any breakout attempt, it will be possible to prevent Iran from acquiring nuclear weapons, despite Iran's failure to come clean about the past and the absence of a more definitive and satisfying conclusion of the PMD issue than what will unfold in coming days and weeks.

Robert Einhorn is a senior fellow at the Brookings Institution. From 2009 to 2013, he was Special Advisor to the Secretary of State for Nonproliferation and Arms Control and a member of the U.S. negotiating team for the Iran nuclear negotiations.

Image: Flickr/President of the European Council