The outlines of a potential agreement between the U.S. and Iran are emerging from the indirect negotiations in Vienna. The two sides will probably succeed in their professed aim of resurrecting the nuclear deal secured by President Barack Obama and renounced by President Donald Trump. But how much that would practically accomplish is questionable.

Washington and Tehran say they’re prepared to return, on a "compliance for compliance” basis, to the terms of the Joint Comprehensive Plan of Action they agreed, with other world powers, in 2015. On the American side, that would mean the gradual lifting of economic sanctions on the Islamic Republic; on Iran’s part, it would involve scaling down uranium-enrichment activities and disposing of new stockpiles.

But turning the clock back is more complicated than it sounds, for both technical and political reasons.