Much of the answer depends on Iran's decision to suspend its weapons programme in 2003. The NIE was dissected by pundits and politicians alike. But no one in the ensuing acrimonious debate seemed to notice a crucial question that had not been asked: how advanced was the programme when Iran supposedly halted it? It seems an important question and one that the intelligence community should have answered. 

The answer came recently, in two separate reports that were leaked to the press. Last March, a German intelligence report was submitted to Germany's Constitutional Court to back the conviction of a German-Iranian businessman accused of supplying Iran with technology for its nuclear programme. The defence had cited the NIE to suggest that the transaction, which occurred in 2007, could not have been used to supply Iran's military programme, given that the latter had been halted four years before. The court upheld the conviction based on the intelligence, which contradicted the NIE — the weapons programme, the German spies said, had never been suspended. A more recent report, published in July in The Times, cited Western intelligence sources as suggesting that Iran had indeed halted its weapons programme in 2003 but only because by then it had been successfully completed.

If the report is accurate, it answers the question the NIE did not address. Iran stopped its nuclear weaponisation programme in 2003 because its strides had far outpaced the enrichment programme. The decision to suspend had nothing to do with the invasion of Iraq or with the much-vaunted secret negotiations between the US and Iran that were ongoing in Paris at the time. It mattered little that IAEA inspectors had started snooping around the recently exposed nuclear installations. Rather, Iran had finished the weaponisation part of the programme before it had completed perfecting a delivery system and mastering the enrichment process.  

Iran's decisions have never been influenced by offers and incentives. The only thing that has ever mattered to Tehran was time. The only reason Iran might still be willing to negotiate is again time: if it still needs time to complete its goal of nuclear weapons capability. US engagement will not change this. Iran can build a bomb, has been busy building one and has never even considered changing its mind.