Iran nuclear talks entered the decisive endgame yesterday with a final round of negotiations which could go right down to a July 20 deadline for a deal.
The accord being sought by Iran and the five permanent members of the UN Security Council, plus Germany, would finally ease fears of Tehran obtaining nuclear weapons and silence talk of war.
In exchange, punishing sanctions on the Islamic republic would be lifted.
With Sunni Islamic insurgents overrunning large parts of Iraq, and Syria in chaos from three years of civil war, a deal could help Tehran and the West normalise relations at a particularly explosive time in the Middle East.
"In this troubled world, the chance does not often arise to reach an agreement peacefully that will meet the needs of all sides, make the world safer, ease regional tensions and enable greater prosperity," US Secretary of State John Kerry said this week.
The so-called P5+1 powers have proposed to Iran a "series of reasonable, verifiable and easily achievable measures", he said, warning Iran not to "squander a historic opportunity".
Iranian Foreign Minister Mohammad Javad Zarif, in a video message, called the talks a "unique opportunity to make history", saying success would allow both sides to address "common challenges" such as Iraq.
In theory, the July 20 deadline could be extended by up to six months, and many analysts believe this is already being negotiated.
But US President Barack Obama, facing midterm elections in November, is wary of doing anything that could be construed by Republicans as giving Iran more time to get closer to having the bomb.