As negotiators collect in Vienna for talks aimed at reviving an intercontinental nuclear settlement with Iran, a single huge concern looms: Has Tehran advanced its nuclear perform so significantly in the previous two years that the 2015 deal can no lengthier be rescued?

Restoring the pact, which positioned boundaries on Iran’s nuclear pursuits in trade for reduction from economic sanctions, is a leading foreign-policy purpose of the Biden administration. Iran’s new president, however, has delayed restarting talks while urgent forward with nuclear perform.