As Iran ponders whether to accept curbs on its nuclear program, it worries less about the possibility of foreign military attack than about the relentless onslaught of economic sanctions that are squeezing its oil-based economy.

U.S. and European officials have said that only tangible progress in the talks due to resume in Baghdad May 23 could convince them not to fully implement measures this summer that will bar Iran from selling oil to Europe and make it harder for Iran to receive payment from remaining customers.  

But it is unclear whether Iran will get the degree of relief it seeks in return for the kind of measures it is likely to embrace. Such a mismatch could jeopardize already uncertain prospects for progress.

If Iran, for example, agrees to stop enriching uranium to 20 percent U-235 — a concession Iran has been floating since last year — the most it might get in return is a brief delay of the European oil embargo and a moratorium on new punishment. Even that is not certain. It is possible that Iran would only receive fuel for a reactor that makes medical isotopes — resurrecting a confidence-building measure that foundered in 2009.