The crucial visit to Tehran by a senior team of from the International Atomic Energy Agency (IAEA) is forcing Iran into a decision that some Iranians say will be their most important since the end to their war with Iraq 15 years ago.
If Iran cannot satisfy the IAEA by the end of October that its nuclear activities are purely peaceful, it could face referral to the UN Security Council and possible sanctions for breaching its Non-Proliferation Treaty obligations.
That could lead Iran along the same path as North Korea, into international isolation and possibly even confrontation with the US or Israel.
That would carry the country years back, and destroy much of the achievements made in recent years.
But the angry Iranian rhetoric which followed the IAEA's adoption of a tough resolution last month has all but faded away.
Khatami has vowed to co-operate
The signs are that the Iranian leadership has decided to swallow its pride and go along with the agency, with reservations and conditions.
"Tehran will continue its co-operation with the IAEA despite the inappropriate resolution it adopted, because we are not worried about the transparency of our peaceful nuclear activities," President Mohammad Khatami was quoted as saying, shortly after the talks began on Thursday.
The IAEA had earlier given Iran a detailed programme of works for the inspections, visits and meetings it wants to carry out this month, as well as some detailed questions it wants answered.
Asked specifically whether Iran would co-operate with that schedule, Iranian ambassador to the IAEA Ali-Akbar Salehi, who is in Tehran for the talks, told the BBC: "Yes. This is our first priority, this is our first option: to continue our co-operation as far as it is possible."
Mr Salehi said Iran would allow further visits and sample-taking at sites where traces of highly enriched, weapons-grade uranium were found by earlier inspections.
But he cautioned that flexibility and co-operation would have to go both ways, and that the IAEA should refrain from pushing for too much.
"We do not want to hinder any process," he said.
"But it is also up to the agency to be prudent and cautious, and not to pose new questions and things that have never been requested before."
Mr Salehi indicated that some "not very serious" splits remained within the Iranian leadership over how far to go in meeting IAEA demands, especially on new elements that he said had arisen.
When the IAEA resolution was passed - after heavy lobbying by the US - on 12 September, many leading Iranian hard-liners pressed publicly for Tehran to break with the Non-Proliferation Treaty and follow the route taken by North Korea.
They seem to have lost out in the behind-the-scenes debate between the country's many power centres, which have concluded that the national interest lies in cooperating with the IAEA - with reservations and conditions.
Iran has made it clear it wants practical recognition of its right to pursue peaceful nuclear energy, including the processing of its own uranium fuel.
That is something the IAEA has asked Tehran to suspend for the time being, pending verification and clarification of its activities.
Tehran reports have said the Iranians are also asking that "political and security sites" should be excluded from scrutiny if they sign up to an Additional Protocol permitting a tougher inspection regime.
Desire to negotiate
The Iranians clearly want to negotiate, but they may find the IAEA officials, bound by a tough resolution, have little authority to make concessions.
"There's not a lot of room for negotiations," said an agency official in Vienna.
That makes the outcome of the initial high-level, political phase of the talks uncertain.
If they succeed in agreeing on a schedule of works, a team of experts in enrichment and centrifuges would begin a busy round of inspections and investigations almost immediately and continue until 31 October.
If they fail, Iran stands to find itself pushed willy-nilly back into the world of international isolation, sanctions and waning relations with trade partners.
"It's a crunch issue in the sense that it will definitely shape the course of Iranian foreign policy for some time to come - there's no doubt about that," says Hadi Semati, professor of politics at Tehran University.
If events tilt one way, international isolation would likely be linked to the ascendancy of the hard-liners at home, while compliance and Iran's further rehabilitation in the world community would favour the reformists.