"Now the difficult part starts"

Enrichment remains another thorny issue. The Obama administration has made clear that it is not prepared to concede up front that Iran has a “right” to enrich uranium.

But the interim agreement makes clear makes clear that a follow-on agreement would provide for a “mutually defined enrichment program with practical limits and transparency.”

So the question is not whether the Obama administration is prepared to allow Iran to continue enriching uranium but rather what constraints the United States and its negotiating partners will insist on in return and how large an Iranian enrichment program they are willing to tolerate.

The interim accord makes clear that such an enrichment must be consistent with “practical needs” and Iran and the United States are likely to have very different ideas of what those requirements would be.

“This, of course, will be one of the central issues in the negotiations for a comprehensive agreement,” said Gary Samore, who served as senior aide on nonproliferation issues on the National Security Council during the Obama administration and is now president of United Against Nuclear Iran, an organization that urges that strong sanctions be imposed on Iran until it further restricts its nuclear efforts.