The agreement is based on a theoretical benchmark: that Iran would need at least a year to produce fissile material sufficient for a weapon, compared with two months or less now. It remains to be seen whether the limits on enrichment and Iran’s stockpile will be judged by independent experts as sufficient to meet that standard.

Both Mr. Obama and Secretary of State John F. Kerry emphasized that many details need to be worked out in talks with Iran between now and the end of June. During that time, the administration will have much other work to do: It must convince Mideast allies that Iran is not being empowered to become the region’s hegemon, and it must accommodate Congress’s legitimate prerogative to review the accord. We hope Mr. Obama will make as much effort to engage in good faith with skeptical allies and domestic critics as he has with the Iranian regime.