The nuclear talks between Iran and the P5+1 powers have reached a critical stage. I am reasonably confident that by next month’s deadline, we can reach a comprehensive agreement that will assure the world that Iran’s nuclear program will remain exclusively peaceful. All that is required is a sober appreciation of the realities faced and a serious calculation of alternatives. Illusions have in the past led to missed opportunities and should not be allowed to ruin the real prospect of the historic deal before us.

When current President Hassan Rouhani and I were leading the Iranian nuclear negotiating team almost 10 years ago, just before the election of former president Mahmoud Ahmadinejad, I presented a proposal to our Western counterparts that contained an array of measures designed by independent, non-Iranian scientists to provide assurances that our nuclear program would remain forever peaceful.

Prodded by the Bush administration, however, our counterparts demanded that we abstain from enrichment until at least 2015, effectively killing the chances of a deal. Their mistaking our constructive engagement for weakness, and opting for pressure and sanctions to gain concessions, led to a change in Iran’s position, both by the ballot box in the 2005 presidential election and the subsequent expansion of Iran’s peaceful nuclear activities.

As we approach 2015, the outcome of past maximalism and obsession with sanctions is clearly evident. In the past 10 years, Iran has gone from 200 to 20,000 centrifuges, our enrichment capacity has risen from 3.5 to 20 percent and the Arak heavy-water research reactor is less than a year from being commissioned.

Nobody can rewind the clock. Sacrifices have been made. Capabilities are vastly different. Knowledge and expertise have been attained. None of this can be wished or negotiated away.

Today, President Rouhani and I are back at the negotiating table, and our commitment to constructive engagement has not changed. We are willing to provide assurances of the exclusively peaceful nature of our nuclear program. Our proposed measures are serious and would make a real difference. But we will not abandon or make a mockery of our technological advances or our scientists, nor would it be prudent or serve the interest of nuclear nonproliferation to expect us to do so.

And we have already delivered. Within 100 days of my being appointed as Iran’s nuclear negotiator, the first nuclear agreement in a decade was concluded with the P5+1. The International Atomic Energy Agency has verified that we have kept up our end of the bargain. Furthermore, the cooperation we now extend to the IAEA has been recognized as the best in years. We are prepared to maintain this trajectory.

It would be tragically shortsighted if illusions were to again derail progress toward a historic achievement. There will be no better time to put an end to the unnecessary nuclear crisis than now, when all sides have much to gain and before the window of cooperation and pragmatic reason closes.

Excuses for once again torpedoing a deal, which can change the shape of our region, can certainly be found. Prominent among them is the myth of “breakout.” For years, small but powerful constituencies have irrationally advanced the idea that Iran can produce enough fissile material for a bomb in months.

While reaching a realistic deal is the best available option for the West to prevent such a remote possibility, it may be instructive to take that phobia at face value. Let’s put it to a logical test. If Iran ever wanted to break out, all IAEA inspectors would have to be expelled from the country. Iran’s program would then have to be reconfigured to make weapons-grade fissile material, which would have to be converted to metal, be molded into the shape required for a bomb and undergo countless other complex weaponization processes. None of these capabilities exist in Iran and would have to be developed from scratch. This would take several years — not a few months.

Even when Iran had the time for this, it did not opt for a bomb. Between 2005 and 2013, when its relations with the West and the IAEA were at rock bottom, Iran had time, little international constraints, relatively relaxed monitoring and enough centrifuges to press ahead toward a bomb. Furthermore, Iran had already paid the price of massive, unjust sanctions that far exceeded those imposed on countries that have developed a bomb.

Despite all this, we did not take a single step toward a nuclear weapon. The 16 security organs behind two consecutive U.S. National Intelligence Estimates, in 2007 and 2012, agreed.

It is ironic that some in the West ignore all of this in favor of projecting the dangerous double myth that Iran needs the bomb to protect itself and is only months away from getting one. It will be even more ironic if this hype torpedoes a deal that is the surest and safest way to preclude proliferation.

Today, we have a unique opportunity in our negotiations with the P5+1 to put in place long-term confidence-building measures, as well as extensive monitoring and verification arrangements, to provide the greatest assurance that Iran’s nuclear program will forever remain exclusively peaceful. To overcome the obstacles to realizing this historic achievement, we must look ahead, but we also cannot ignore the lessons provided by the past. Comprehension of how the cycle of lost chances has been propelled by illusions is important. Taking action to exit this cycle is crucial.

As we enter the crossroads of turning the interim nuclear deal into a comprehensive solution, I urge my counterparts to reciprocate our willingness to address concerns about our capabilities with appreciation of our demand for our rights, dignity and respect. Most of all, I urge them to refrain from allowing illusions to derail the march toward ending an unnecessary crisis and opening new horizons.