Any evaluation of the Iranian nuclear deal, also known as the Joint Comprehensive Plan of Action or JCPOA, must consider three different time periods, each of which is roughly a generation.

First, of course, is the initial 10–15 year period of the agreement itself, in which extraordinary restrictions will be placed on all of Iran’s nuclear activities: from uranium mines to centrifuge production plants; from the configuration of its nuclear reactors to the operation of its enrichment facilities; and from the size of its uranium stockpile to the level of enrichment—all of this and more will be under the constant supervision of the International Atomic Energy Agency (IAEA). It is correct to label these restrictions extraordinary. No other nation in the 47-year history of the Nuclear Non-Proliferation Treaty (NPT) has ever voluntarily agreed to such intrusions into its nuclear activities.

Protester Kayla Marks of Hollywood, Florida holds a sign before U.S. Vice President Joe Biden met with Jewish community leaders at the David Posnack Jewish Community Center in Davie, Florida, September 3, 2015. Joe Skipper / Reuters It is also correct to praise the complex and interlocking terms of the agreement. One creative element is the “snap back” provision, which permits the re-imposition of sanctions by the United Nations in case of serious breach of the agreement, with no veto right. The agreement also provides a special supervised channel for Iranian trade in peaceful nuclear materials and technical oversight provisions that have never been applied to any other country that is party to the NPT. One of the chief U.S. negotiators repeatedly compared the agreement to a Rubic’s Cube, since every part depends on every other part.

As the limits and terms of the agreement came to be better understood, even its harshest critics generally acknowledged that they would dramatically raise the barrier for any Iranian attempt to break out. There were some remaining questions about enforcement mechanisms, such as fears that access to a suspect site could be delayed by up to 24 days or that access to Iranian military sites would be impeded. However, assurances from Yukiya Amano, the director general of the IAEA, and from U.S. Secretary of Energy Ernest Moniz, one of the world’s leading nuclear physicists, who was intimately involved in the final negotiations, satisfied most of those concerns.

By 2030 and certainly by 2050, Ayatollah Ali Khamenei, now approaching age 80, will no longer be Supreme Leader of the Islamic Republic of Iran. Similarly, most (if not all) of the leading political and clerical figures in Iran who carried out the 1979 revolution and who have governed Iran ever since, will no longer be politically active.

By the end of debate, the key question was less about the operation of the agreement itself and more about what would happen when the extraordinary restrictions were removed after 10–15 years. That is, what would happen during the second generation, beginning in roughly 2030.

Trying to look ahead that far, especially in a country such as Iran that is undergoing significant demographic and social changes, is impossible. There are, however, a few things that one can say with some certainty. By 2030 and certainly by 2050, Ayatollah Ali Khamenei, now approaching age 80, will no longer be Supreme Leader of the Islamic Republic of Iran. Similarly, most (if not all) of the leading political and clerical figures in Iran who carried out the 1979 revolution and who have governed Iran ever since, will no longer be politically active. And the United States could have changed presidents two or even three times.

Britain's Foreign Secretary Philip Hammond leaves a Royal Air Force airplane after arriving at Mehrabad Airport in Tehran, Iran August 23, 2015. Britain reopened its embassy in Tehran on Sunday, a historic step in the thawing of relations between the Islamic state and Western powers, a Reuters witness said. Darren Staples / Reuters Assuming that the nuclear agreement holds throughout the first generation, moreover, Iran will have experienced years of reduced sanctions, potentially greater engagement in world commerce, and interaction with the international community. That is no guarantee of liberalization, of course, but at least some people believe that this agreement offers the best chance of such an outcome. They may or may not be correct; it is impossible to say where Iran will be in 2030 or 2050—just as it is impossible to predict the future of Saudi Arabia or Syria, Iraq, and ISIS.

Many critics of the JCPOA warn that as soon as the agreement and its extraordinary restrictions expire, Iran will make a sudden rush to a bomb. Some opponents of the agreement even go so far as to say that, in fact, the deal, which recognized Iran’s right to peaceful enrichment, ensures that Iran will get the a bomb. This view overlooks—or simply dismisses—the fact that, as part of the JCPOA, Iran will have ratified the Additional Protocol of the NPT, which means that it will be subject to inspection and oversight by the IAEA in perpetuity. No country that has accepted the Additional Protocol has ever launched a nuclear weapons program.

There is a first time for everything, of course, but the best predictor of Iran’s future behavior is its past, specifically the past 15 years or so of our experience with Iran. And that brings us to the final generation worth reviewing.

For nearly a quarter of a century, we have heard frequent cries of alarm that Iran would have a nuclear weapon in two years or three years or five years. The warnings have always been wrong.

The critique of the agreement with Iran starts with the assumption that Iran is determined to get a nuclear weapon and that the only way it can be stopped is by depriving it of one key part of the nuclear cycle: enrichment. The only way do that, the argument goes, is to sharply increase sanctions, maximize pressure in all forms, and demonstrate a clear willingness to use force if necessary to force Tehran to submit. What the critics fail to mention is that this is precisely the policy that the United States pursued over the decade and a half in which Iran made most progress toward the bomb.

After the overthrow of Saddam Hussein in Iraq in 2003, after U.S. President George Bush labeled Iran as part of the “Axis of Evil,” and with U.S. forces encircling Iran in Afghanistan, Iraq, and the Persian Gulf, Iran made an offer that was not dissimilar to the starting point for the present agreement. The 2005 offer included what Iran called “concrete assurances” that Iran would not build a nuclear weapon, but it was far less detailed than the present agreement. The entire U.S. intelligence community later concluded with high certainty that at the time Iran made its move, it had already ceased any work that it might have been doing on nuclear weapons for two years.

Iranians celebrate on the streets following a nuclear deal with major powers, in Tehran July 14, 2015. Reuters The offer was developed by Hassan Rouhani, who was then the secretary of the Iranian Supreme National Security Council, and conveyed to the West by Javad Zarif, who was at that time the Iranian ambassador to the United Nations. It insisted that Iran should retain the right to a full nuclear fuel cycle, including enrichment, although Tehran was willing to cap its centrifuges at only a few hundred. The Bush administration refused the offer on the grounds, as I was informed in a New York meeting with U.S. and British officials, that “even one centrifuge turning was one too many.”

In subsequent years, the United States used all of its influence to bring maximum international pressure on Iran to cease its nuclear development. Eventually the United States was able to persuade its allies and others to impose sanctions on Iran to force it to cease its enrichment and other aspects of its nuclear program. This process reached its zenith under President Barack Obama, whose administration promoted and managed the most comprehensive array of sanctions ever imposed on any country that had not suffered a military defeat. At the same time, voices in Israel and the United States were calling for direct military action against Iran.

While this was happening, Iran steadily ramped up its nuclear capacity. Between 2005 and 2013, Iran increased its number of centrifuges from near zero to approximately 20,000. Its stockpile of enriched uranium increased from zero to more than eight tons, including more than 200 kg (440 pounds) of uranium enriched to nearly 20 percent, which is within easy reach of the 90+ percent required for weapons grade. Under the relentless pressure of sanctions and threats of military attack, Iran became a latent nuclear power, a country that could potentially create enough fissile material for multiple nuclear weapons in less than a year.

The dominant narrative in the United States is that U.S. sanctions brought Iran to the negotiating table. But, in private, Iranian negotiators claim that it was their determined drive to develop a nuclear capacity—at great cost—that brought the United States and others to the table. Both may be correct.

Today the authors of Iran’s original offer to the Bush administration are the president and foreign minister of Iran. They have negotiated a historic agreement with the world’s great powers that severely constrains any possibility of an Iranian nuclear weapon but which maintains the principle of Iran’s right to enrichment. Many of their own countrymen question the cost that was paid in the process.

For their part, U.S. critics of the agreement must at least address the question of why Iran, when it was very close to having a nuclear weapons capability, chose instead to negotiate and to postpone any drive toward the bomb for at least a generation and probably forever. They should also ask why in 2030, absent a war, Iran would choose to go back to where it was, with far fewer constraints on its behavior in 2013.

For nearly a quarter of a century, we have heard frequent cries of alarm that Iran would have a nuclear weapon in two years or three years or five years. The warnings have always been wrong, but by sheer repetition, this mantra has acquired the ring of truth. The terms of the JCPOA and its proper enforcement provide a unique opportunity to break the cycle of defeat.