What if it all goes wrong? The US and its allies have spent months painstakingly assembling a coalition behind imposing tougher sanctions on Iran to contain its nuclear ambitions.
Concessions have been offered along the way: last year, the US tried to secure Russia's support by going so far as to tear up its plans for missile defence in Europe.
The drive to maximise pressure on Iran by any means short of military action has absorbed so much effort that it has become the largest single priority of the Obama administration's diplomacy.
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