Iran nuclear stakes high as 'end' talks begin
Few people believe Iran and the six world powers it is negotiating with will be able to reach an agreement by the 24 November deadline to resolve the crisis over Tehran's nuclear ambitions.
But if they do - and that is a big "if" - one of the most dangerous crises in the Middle East may finally be defused.
That is why the discussions that start in Vienna on Tuesday are important for all of us.
In theory, a comprehensive agreement would mean that Iran, even if it wanted to, would not be able to build a nuclear bomb, now or in the foreseeable future.
It would lead to sighs of relief from Iran's neighbours and beyond, particularly Israel.
The serious threat of yet another war that has been hanging over the region for nearly 10 years would be removed.
Neither the US, nor Israel, would have reason to attack Iran. Israel would cease to feel its very existence was in imminent danger.
An agreement would remove the incentive for Middle Eastern countries to rush into an arms race to build their own nuclear weapons. It is widely accepted that Israel already has nuclear weapons, though it neither confirms nor denies this.
An agreement might help improve relations between Iran and the US - at loggerheads for 35 years. Iran might even actively join efforts to fight the Islamic State group in Iraq as well as Syria, where it might be persuaded to take a more constructive role in bringing the Syrian civil war to an end.
An agreement would mean many of the sanctions that have isolated Iran and have crippled its economy could begin to be lifted - excellent news for beleaguered Iranians who have seen the value of their money halved in the past few years together with the country's income from oil exports.
Iran would come in from the cold. Banks in the Persian Gulf countries as well as in Europe and the Far East would begin dealing with Iranian banks, reopening normal channels of commerce. Iran would be open for business again.
An agreement would open up a market of 80 million people. Many countries would be able to resume purchasing oil from Iran. Big industries would be free to sell goods and provide shipping, insurance and other services.
But despite all this, most observers do not think a deal is likely.
"Big gaps remain and we don't know whether we can close them," said US President Barack Obama recently.
Iranians are saying much the same.
The six world powers want to close all paths that Iran might take towards building a bomb. That means Iran must reduce the size of its uranium enrichment activities.
The number of centrifuges and the stockpile of enriched uranium must be reduced to a level that would mean Iran needed six months to a year to have enough enriched uranium to make a bomb, if it decided to do so - the so-called "break-out" time.
The regime of international inspections has to be greatly strengthened. The heavy water reactor near the central Iranian city of Arak must be redesigned to reduce drastically its ability to produce plutonium - another route to a nuclear bomb.
With its present design, when it goes on stream in a couple of years, it will be able to produce enough plutonium for two nuclear bombs a year. Iran denies it has any plans to build nuclear weapons.
It says it will not roll back its programme - a national scientific feat by Iranian scientists - insisting it is for peaceful purposes, for manufacturing fuel needed for planned future nuclear power plants, for producing medical isotopes and for research.
The rhetoric Iran's religious leaders have been using to defend their nuclear programme at home makes it very difficult for Iran to downsize its programme.
The hardliners who are opposed to President Hassan Rouhani have made the nuclear programme their rallying cry by appealing to the nationalist sentiments of many Iranians.
They may have the support of Iran's Supreme Leader, Ayatollah Ali Khamenei. They warned recently that the government of President Rouhani was about to sell out the achievements of Iranian scientists.
Amongst those who are worried outside Iran is Israel's Prime Minister Benyamin Netanyahu.
He is worried that the US administration, eager for a foreign policy success, is ready to make what he calls a bad deal - a deal that keeps Iran's ability to make a bomb intact, and at the same time removes the key sanctions that brought Iran to the negotiating table in the first place.
France is also worried, according to Foreign Minister Laurent Fabius, that Iran has not moved an inch in its position.
And Saudi Arabia has said if Iran is allowed to retain enrichment facilities, it intends to develop similar facilities of its own.
'Rubik's Cube' of issues
In recent months multilateral talks have gradually developed into bilateral talks between Iran and the US - the two main sides in the negotiations.
After the US mid-term elections, President Obama now has to sell any potential deal to a Congress dominated by Republican opponents who are deeply suspicious of Iran.
US negotiators have to be firm and yet conciliatory, and Iranian negotiators have to show that they have not bowed before the Americans nor have they agreed to roll back Iran's programme.
Sanctions have to be lifted but how quickly? Iran wants them lifted in one go, but world powers want to make sure Iran will abide by any agreement that is reached.
In short, in the words of top US negotiator Wendy Sherman, it all adds up to a Rubik's Cube of technical and political issues that still have to be solved.