John Kerry flew to Geneva and, at 3 A.M. Sunday—Swiss time—got the signatures he needed on a deal to stop the momentum of Iran’s nuclear program. The parties to the agreement are Iran and the five members of the Security Council plus Germany. Israel isn’t a part of it, and Benjamin Netanyahu doesn’t like it one bit. (“A historic mistake,” he said.) That doesn’t mean it isn’t good for Israel (or even that all Israelis agree with Netanyahu; they don’t) or for the rest of us. “We have a real opportunity to achieve a comprehensive, peaceful settlement, and I believe we must test it,” President Obama said Saturday night; and, in terms of the promise of the deal, he’s right. The terms are for six months: Iran has to stop producing uranium enriched beyond a certain degree, stop expanding its enrichment capacity with new centrifuges, and stop using a lot of the centrifuges it has.

“Simply put, they cut off Iran’s most likely paths to a bomb,” Obama said, in return for “a new path toward a world that is more secure.” One path is more appealing than the other.

In return, Iran gets what President Obama called “modest relief” from sanctions. Mostly, this means letting Iran have access to some of what is actually its own money, frozen in foreign bank accounts. (The Washington Post summarizes the terms.) There is also a commitment not to impose new sanctions during the six months. That will mean some political work on the part of the Administration, since several Congressional Republicans are already angry about the deal—Congressman Mike Rogers, told CNN that it was a “mistake” that might have just “encouraged more violence”— and some Democrats, too, will be skeptical. (Senator Chuck Schumer said in a statement that he was "disappointed.") But it should be do-able.

The most important aspect, though, might be the oversight requirements. There will be daily inspections and deep access to sites. This helps if you want to stop a country from secretly developing nuclear weapons. But it also helps if you want to open that country up, to ratchet down hostility and check the many ways Iran can destabilize a region that has troubles enough, to help make it a different place. Iran has a new President, Hassan Rouhani, who, Laura Secor wrote in the magazine last week, “is a pragmatist who came to power, in part, on the strength of disaffection, both popular and élite, with the confrontational foreign policy of his predecessor.” The regime’s rhetoric is still violent, its policies destructive—in Syria for example. One doesn’t want to be starry-eyed. And Sunday morning there was back-and-forth over the abstract “right” to enrichment. But, Secor notes, “Rouhani has committed himself to finding a quick resolution to the impasse, and the cumbersome, fractious machinery of the Iranian state has backed him with an unusual unity of purpose. … Gone are the grandiose diatribes and the repetitive talking points of the Ahmadinejad years.” It would be self-destructive on America’s part not to seize this moment.

Kerry is certainly trying. And he appears to have gotten his fingers on the edge of something that could be historic. Now the task is to not let go. “I know there are some people who will say this is imperfect,” Kerry, who deserves credit for giving himself hard tasks, said in Geneva. If so, he said, those people needed to tell him what the alternative was. Six months is a window of the sort that we haven’t had on the nuclear question in a long time. What can the Administration get through before it closes? Kerry said on CNN that it would be “exploring and testing … we do that with eyes absolutely wide open.” He added, “none of this is based on trust.”

Photograph by Denis Balibouse/Reuters.