Iran and six world powers descend on Vienna today seeking to nail down a mammoth nuclear deal, six days before a deadline with differences still considerable despite months of negotiations.
Such an accord could not only consign to history one of the 21st century's most intractable geopolitical conundrums by easing fears once and for all that Iran might build a nuclear bomb.
It could also silence talk of war, put Iran and the West on the road to normalised relations after 35 years in the deep freeze and give US President Barack Obama a rare foreign policy success.
Hardliners in both the United States and Iran are putting their negotiators under pressure not to give too much away, however, and it is far from certain that a deal can be done.
"There's still a big gap. We may not be able to get there," Obama warned last Sunday.
Iran's arch foe Israel, widely assumed to have a formidable nuclear arsenal itself, is also watching closely, as are Sunni Gulf monarchies uneasy about any US rapprochement with Shiite Iran.
The United States, China, Russia, Britain, France and Germany (the P5+1) want Iran to scale down its nuclear programme to make it virtually impossible for Iran to assemble an atomic bomb.
Iran, which says its nuclear aims are exclusively peaceful, wants painful sanctions lifted and a recognition of its "right" to a peaceful nuclear programme.
On November 24 last year, after moderate Hassan Rouhani became president, Iran and the P5+1 secured an interim agreement.
However, they missed a July 20 deadline to reach a comprehensive accord, giving themselves four more months, which expires this coming Monday.
US Secretary of State John Kerry, due back in Vienna later this week, said at the time that the talks were "the best chance we've ever had to resolve this issue peacefully".
And now, says chief US negotiator Wendy Sherman, it is "time to finish the job".
