By Jon Leyne
BBC News, Tehran
Iran's first reaction to the best news it had heard for a long time was - silence.
Questions remain about whether Iran needs nuclear power at all
For more than 12 hours, state media did not even mention the new US intelligence assessment that said Iran no longer had an active nuclear weapons programme.
It was almost as if it was too good to be true. The authorities simply could not believe their ears.
Now they have regained their balance, the mood in Tehran is little short of triumphalist.
The foreign ministry says the new assessment shows that everything US President George W Bush says on the nuclear issue is "unreliable and fictitious".
Iranian TV has been covering the report full time, with lengthy discussions all devoted to the theme of "we told you so".
Enriched uranium concerns
But what has really changed?
To hear Mr Bush's recent remarks warning of the threat of "World War III", you would think Iran was putting the finishing touches to its first nuclear warhead.
But the case against Tehran has never relied on such alarmist assessments.
The argument the West puts is that Iran is developing a technology - uranium enrichment - that could be used to help produce a nuclear weapon at some date in the future. And that danger remains the case whatever Iran's current intentions.
There is the question of why Iran developed this technology in secret.
And there is the larger issue of why Iran needs peaceful nuclear power at all, when, according to the oil company BP, it possesses the world's largest combined reserves of oil and gas.
Indeed, while Iran is working to produce enriched uranium, which it says is for peaceful purposes, it is pleading with Moscow to provide it with nuclear fuel for the reactor just being completed at Bushehr.
In fact, some diplomats here believe any enriched uranium Iran produces is not and could not be used at Bushehr, for both technical and political reasons.
But even if Iran does not need peaceful nuclear power, it does not follow that this programme has a military end.
Indeed, the new US intelligence assessment only lends strength to what many Western diplomats here already believe, and Iran consistently claims. There is no active Iranian military nuclear programme.
If not military, nor entirely peaceful, then why?
As one Iranian analyst once said to me, only half joking, if the Americans were not opposed to it, the Iranians would stop their nuclear programme tomorrow.
In other words, it is a power play.
By defying Washington, the Iranian regime is using the nuclear issue to strengthen its hand, just in the way North Korea did so successfully.
Conversely, there are those in Washington, probably including Vice President Dick Cheney, certainly including former UN ambassador John Bolton, who would like to use the nuclear issue to unseat the Iranian regime.
If that is the case, then whether Iran intends to produce a nuclear weapon is, in a strange sense, almost academic. It is the challenge to US power that is all important.
Iran's main aim, according to this interpretation of events, is to secure the regime and strengthen its power in the world.
The US report could strengthen President Ahmadinejad's hand
This is exactly the opposite of Israel's feared scenario, which is that Iran is building itself up for one last suicidal strike at the "Zionist entity".
This view of the nuclear programme as a "power play" is strengthened by the latest US intelligence assessment. But the assessment itself does not necessarily end the game. It just changes the balance of power.
After this report, the chance of military action by the US, already considered fairly small, becomes even smaller. It will also make it harder for Washington to secure the tougher sanctions it has already called for.
The new assessment will strengthen Iranian President Mahmoud Ahmadinejad's hand, at least in the short term. He will no doubt claim he faced down the "Great Satan" and won.
But in reality, this is just one more roll of the dice, another twist in the confrontation between Washington and Tehran that has been going on since the Islamic revolution in 1979 and shows no sign at all of ending.