ran Agrees to Detailed Nuclear Outline, First Step Toward a Wider Deal

Federica Mogherini, left, the European Union’s foreign policy chief, and Foreign Minister Mohammad Javad Zarif of Iran

Iran and the United States, along with five other world powers, announced on Thursday a surprisingly specific and comprehensive understanding on limiting Tehran’s nuclear program for the next 15 years, though they left several specific issues to a final agreement in June.

After two years of negotiations, capped by eight tumultuous days and nights of talks that appeared on the brink of breakdown several times, Secretary of State John Kerry and his Iranian counterpart, Mohammad Javad Zarif, announced the plan, which, if carried out, would keep Iran’s nuclear facilities open under strict production limits, and which holds the potential of reordering America’s relationship with a country that has been an avowed adversary for 35 years.

Mr. Kerry and Energy Secretary Ernest J. Moniz, a nuclear scientist who played a crucial role in the last stages of the negotiations, said the pact satisfied their primary goal of ensuring that Iran, if it decided to, could not race for a nuclear weapon in less than a year, although those constraints against “breakout” would be in effect only for the first decade of the accord.

President Obama, for whom remaking the American relationship with Iran has been a central objective since his 2008 campaign, stepped into the Rose Garden moments later to celebrate what he called “a historic understanding with Iran.” He warned Republicans in Congress that if they tried to impose new sanctions to undermine the effort, the United States would be blamed for a diplomatic failure.

He insisted that the deal “cuts off every pathway” for Iran to develop a nuclear weapon and establishes the most intrusive inspection system in history. “If Iran cheats,” he said, “the world will know it.”

Under the accord, Iran agreed to cut the number of operating centrifuges it has by two-thirds, to 5,060, all of them first-generation, and to cut its current stockpile of low-enriched uranium from around 10,000 kilograms to 300 for 15 years. An American description of the deal also referred to inspections “anywhere in the country” that could “investigate suspicious sites or allegations of a covert enrichment facility.” But in a briefing, American officials talked about setting up a mechanism to resolve disputes that has not been explained in any detail.

In a move not seen since before the Iranian revolution in 1979, and to the surprise of many in both countries, Iranian government broadcasters aired Mr. Obama’s comments live.

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