By Gordon Corera
BBC security correspondent
In the coming days, Iran is expected to make what is being billed as a major announcement on its nuclear programme to coincide with the anniversary of the Iranian revolution.
Tehran says its nuclear programme is purely civilian
But just how close is Iran to mastering nuclear technology?
Both Iran and some of its critics may have their own reasons for exaggerating the progress - but the real truth is hard to establish.
In its announcement, Iran may claim to have begun large-scale industrial enrichment of uranium.
But any statement is likely to be as much about political positioning as real technical progress, according to nuclear analysts.
The announcement may focus on work Iran has conducted in installing two cascades of more than 300 centrifuges in an underground industrial size plant at Natanz with the aim of moving towards a total of 3,000 machines.
The centrifuges are used to enrich uranium. This is in addition to two existing cascades in a pilot plant above ground.
With US troops so close to Iran's borders, a small event could easily ignite a wider escalation and even trigger an 'accidental' war
But Iran's plan to initially run 3,000 centrifuges before moving towards an ultimate goal of 54,000 has run into obstacles and delays and is well behind target. Even the cascades in the pilot plant have seen problems.
However, once Iran has mastered the technology of enrichment and the ability to enrich gas at high speeds in a centrifuge then transferring it to a larger scale presents a lesser challenge.
Uranium enriched to around 5% can be used as nuclear fuel, but if it is enriched to around 90% it can be used in a weapon.
Over the years, some of the problems with the programme seem to be due to Iran's own mistakes.
Diplomats have been shown the Isfahan nuclear plant recently
For instance, one of the top figures in the programme has talked of how in the early days, those assembling the centrifuges did not wear cloth gloves.
As a result, tiny beads of sweat would be transferred to the rotor which spins inside the centrifuge.
This almost imperceptibly increased the weight of the rotor which then unbalanced the centrifuge when it started to spin, causing it to "explode".
Iran also was thought to have had problems with the purity of the uranium hexafluoride which is fed into the centrifuges, although its scientists now say this has been solved.
But the problems may also be due to more shady activity by others.
Over a number of years, both US and Israeli intelligence are believed to have covertly passed flawed parts and equipment to Iran to cause technical difficulties and slow the Iranian programme down.
In one event last April, according to Iranian press reports, the explosion of another set of centrifuges was attributed to problems with the power supply.
Ayatollah Ali Khamenei has warned the US against attacks on Iran
The supply needs to be kept precise and constant to ensure the centrifuges spin at the correct speed but Iranian scientists said that on this occasion the power supply might have been "manipulated" which may imply they were sabotaged.
It is possible that some of the electrical parts for Iran may have come through the Turkish end of the network run by Pakistani scientist AQ Khan which also supplied electrical components to the Libyan nuclear programme.
By the end of the network's activity in early 2004, it had been penetrated by British and American intelligence with some of the suppliers turned as agents.
Recent reports have also questioned whether the death in January of a 45-year-old Iranian scientist, Ardeshire Hosseinpour, might have been the result of an operation by Israel's intelligence service, Mossad.
Hosseinpour had been involved in the enrichment programme, but Iranian reports have denied that his death was due to anything other than natural causes.
Mossad is widely believed to have been behind a campaign of killings and intimidation targeted at the Iraqi nuclear programme and some of its suppliers in Europe in the early 1980s, but this has never been definitively proven.
Arguably it is human expertise in the form of trained scientists rather than equipment which is the most important element of a nuclear programme.
Whether or not there has been extensive covert activity directed at Iran (and by definition it is hard to discern the truth), the variety of technical problems mean that its hard to know if Iran is actually far away from mastering nuclear technology or relatively close to it and thereby able to make the relatively short journey from "peaceful" civilian technology towards manufacturing nuclear material for a bomb.
The problem is that there remain many "unknowns" when it comes to the Iranian programme.
One of the most important is exactly how much help Tehran received from the Khan network.
The network first sold centrifuge designs to Iran in 1987 and provided on-off help for more than a decade after, including parts and designs for more advanced machines.
But international investigators remain unsure that they have an understanding on the full extent of the assistance, not least because no-one outside Pakistan has been able to question Khan directly whilst he remains under a form of house-arrest in Islamabad.
The biggest question surrounds the more advanced P2 centrifuge design that Khan passed to the Iranians.
Iran initially said it had conducted little work on the design but last year Iranian President Mahmoud Ahmadinejad said that Tehran was working on the machine (which would be far more efficient than the model in Natanz).
However, the International Atomic Energy Agency (IAEA) has not been provided any information on such work.
If Iran was able to run a parallel, second enrichment program which it had managed to keep secret, then many of the estimates of how far Iran was from mastering the technology might be way of the mark. But this remains an unknown.
The degree of uncertainty can cut the debate over action against Iran in both directions.
Some voices argue that Iran remains at least five years away from nuclear weapons capability, and US intelligence estimates have consistently pushed back when that might be - so some argue there is no rush.
Other hawkish and pessimistic voices argue that Iran could soon master the technology and the time-frame for action lies this year.
Israel is keen to emphasise that it sees the shorter time-frame as the valid one and is willing to take action.
The US has been playing down its willingness to engage in military action but is currently pushing the Europeans to squeeze Iran financially.
But conflict between the US and Iran is still possible.
President Ahmadinejad is facing his own domestic problems with mounting criticism of not just his approach to foreign policy and the nuclear issue but also his failure to deal with economic concerns at home.
This could lead to other power centres in Iran forcing him to back down but could also encourage him to take a harder line on the nuclear programme in order to try and rally support.
At the same time, Washington has been increasing the pressure over Iran's alleged involvement in Iraq.
With US troops so close to Iran's borders, a small event could easily ignite a wider escalation and even trigger an "accidental" war - although conspiracy theorists might argue that there are some in both Tehran and Washington who would like to engineer just such a confrontation and blame the other side.