The inking of the additional protocol to the nuclear Non-Proliferation Treaty at the headquarters of the International Atomic Energy Agency (IAEA) in Vienna later in the day will be nothing short of a dramatic turnaround for the Islamic republic.
Iran is, after all, a country that exists under a veil of secrecy and whose leaders are prone to regular bouts of paranoia over perceived conspiracies orchestrated by arch-enemies Israel and the United States. Both countries accuse Iran of secretly developing nuclear weapons under the cover of generating electricity.
"It's a very important step and this was not an easy decision for Iran," a senior Western diplomat here told AFP. "The idea of letting foreigners sniff around nuclear sites must have been a bitter pill to swallow."
Iran had been resisting pressure to sign the additional protocol. This is hardly surprising given the humiliating inspections regime imposed on Iraq over the past decade, on top of which were allegations that UN teams contained US or Israeli agents.
The argument here was that inspectors could violate national sovereignty and probe sites that are crucial to the defence of the country, lumped by US President George W. Bush into an "axis of evil" along with North Korea and the ousted regime in Iraq.
But under pressure to come clean and allow the IAEA to verify Iran's assertion the programme was purely peaceful, Iran was left with little choice, even though some prominent hardliners called for pulling out of the NPT altogether.
"The alternative would have been shutting itself off and running the risk of UN sanctions and even conflict. In effect, the protocol stands in the way of the United States and Israel being able to use the nuclear issue to legitimise an attack," the diplomat explained.
"If Iran had refused, its denials would have seemed rather hollow," the diplomat added, echoing IAEA Director General Mohamed ElBaradei's assertion that greater inspector powers were needed to verify Iran's claims.
Iran finally buckled when the IAEA threatened to refer its concerns to the UN Security Council, which would have left Iran vulnerable to economic, diplomatic and other sanctions.
There was also speculation that Israel -- and even its international guardian the United States -- were planning military strikes against nuclear facilities here.
The U-turn came in October during an unprecedented visit by the foreign ministers of the European Union's big three -- Britain, France and Germany. Iran agreed to sign the additional protocol, hand over full details of its activities and suspend uranium enrichment.
And the continued cooperation has even been given the approval of supreme leader Ayatollah Ali Khamenei, easing fears that ratification of the protocol could fall victim to reformist-conservative bickering.
After Iran handed over a dossier on its activities in late October, the IAEA condemned what it said was 18 years of covert activity by Iran.
The face-saving spin put on the climbdown and the slapped wrists here was that in return, Iran had secured its right to generate nuclear energy -- a programme the US has been trying to stop altogether.
Iran also won assurances that the IAEA was not intent on violating Iranian sensibilities. As one IAEA source put it, "we're not going to be going in and kicking down doors".
While the signing may ease the war of words between Iran and its critics, the fears that Iran could one day get the bomb are far from buried.
"Diplomatically it is very important, but there are longer-term issues. In particular the nuclear fuel cycle," explained a European diplomat involved in the recent British-French-German brokering.
This relates to Iran's suspension of uranium enrichment, demanded by the IAEA but said here to be only temporary. And if Iran does eventually master the entire nuclear fuel cycle, it could be just months away from being able to develop nuclear warheads while at the same time under IAEA safeguards.