John F. Kerry, a visiting distinguished statesman at the Carnegie Endowment for International Peace, was U.S. secretary of state from 2013 to 2017.

If the United States breaks with the International Atomic Energy Agency, the six other signatories and the conclusions of our own State Department by decertifying Iran's compliance with the nuclear agreement, the deal's fate will rest with Congress under the terms of the 2015 Iran Nuclear Agreement Review Act. It would be facing a decision about America's security, not a referendum on President Trump or former president Barack Obama.

Having cast dozens of arms-control votes as a senator — judging not whether they were perfect, but whether we were better off with them — I want to take those who may soon cast a similar vote "into the negotiating room" to explain the product we negotiated to close Iran's pathways to a nuclear weapon, and why it is so important to keep the agreement in place.

Context matters. When I first met with Iran's foreign minister in September 2013, Iran had mastered the nuclear fuel cycle, had built a uranium stockpile that could be enriched to make 10 to 12 bombs, and was enriching just below weapons-grade. It was moving rapidly to commission a heavy-water reactor capable of producing enough weapons-grade plutonium for an additional bomb or two annually. In other words, Iran was already a nuclear-threshold state.

We spent thousands of hours negotiating to get it right, even though Iran's break-out time to produce enough fissile material for a bomb was just a few months. The United States had, through painstaking diplomacy, marshaled our European allies and reluctant countries — including China, Russia, India and Turkey — to implement crippling sanctions on Iran, but even that hadn't stopped it from speeding ahead from a few hundred centrifuges to thousands. Only negotiation would freeze and roll back the program.

Some ask why our agreement didn't stop Iran's destabilizing behavior, including its support of Hezbollah and the brutal Assad regime in Syria. It's a good question with good answers: We were not going to bargain away certainty on the nuclear issue for anything else; as France said, there would be no "quid pro quo." We had deep disagreements with Iran and zero trust, hadn't negotiated with them since 1979, and were on a collision course toward military action as the countdown clock on break-out ticked down.

The world was united on one issue alone — Iran's nuclear capability. We could not have achieved unity or held the sanctions regime together if we added other issues. But we believed it would be easier to deal with other differences with Tehran if we weren't simultaneously confronting a nuclear regime.

We knew that any agreement would be scrutinized by critics who 20 years ago witnessed the United States reach a deal with North Korea that fell apart. We internalized those lessons. The agreement with North Korea was four pages long and only dealt with plutonium. The agreement with Iran runs 159 detailed pages, applies to all of Tehran's potential pathways to a bomb, and is specifically grounded in the transparency rules of the IAEA's Additional Protocol, developed with the North Korea experience in mind. No country has gone nuclear with the Additional Protocol in place. It's that intrusive. We insisted it be a bedrock of the Iran agreement.

What did we achieve? For one thing, contrary to some reports, it was Iran that had to pay up front. Before Iran received a dollar of sanctions relief, the IAEA confirmed that the country had eliminated 97 percent of its uranium stockpile, destroyed the core from its Arak reactor (which blocked the production of weapons-grade plutonium), ripped out more than 13,000 centrifuges, halted uranium enrichment at the underground Fordow site, and opened its program to intrusive monitoring. In eight consecutive reports, the IAEA has confirmed that it's working.

Much attention has been focused on the agreement's "sunset provisions." That is a misnomer for an agreement that has provisions lasting 10, 15, 20 and 25 years, with the most important ones lasting forever. That said, nearly all arms-control agreements contain time elements, which is why so many result in follow-on accords, once confidence is built on both sides.

We were comfortable because the cap on Iran's low-enriched uranium stockpile remains in place until 2030. It is impossible to produce a nuclear weapon with 300 kilograms of low-enriched uranium.

We were also comfortable because the unprecedented monitoring and verification measures we achieved never expire. Because of the permanent IAEA inspections, the world would know if Iran were foolish enough to seek a bomb.

Fundamentally, it seems irrational to leave an agreement that's working today out of a fixation on potential growth of Iran's nuclear program more than a decade from now, when such growth could happen tomorrow if we unravel the agreement. We'd be back where we were before, only way worse, with the United States isolated, not Iran.

We maintain leverage by sticking with the agreement, and European foreign ministers tell me that they would join us in confronting other Iranian misdeeds. What leverage do we gain by walking away when we know Iran is complying? We lose our close alignment with our allies. We empower Russia and China. We hand Iranian hard-liners a victory and send a message to any country considering a negotiation with us that, when politics intervene, the United States doesn't keep its word. Moreover, sticking with the deal means we don't jump back in the barrel headed toward military conflict with Iran, and we can focus on North Korea's white-hot nuclear threat today.

The agreement the world forged to stop Iran from ever acquiring a nuclear weapon reflected our best judgment about achieving that solitary goal. It was not a wish list we could impose, but the result of a negotiation. We based our conclusions on verification, not trust. In every way the world can measure, it is working.