For most people, Washington's second thoughts about the seriousness of the Iranian nuclear "threat" will come as a great relief.
Those in the Bush administration who appeared bent on forcing a military confrontation with Tehran some time next year will now face greater difficulties in making their case. George Bush's "third world war" is off the agenda - at least for now.
The surprise reversal (pdf) in US official thinking, embodied in the CIA's declassified National Intelligence Estimate (NIE), has also opened up a rare chance for substantive dialogue with Tehran ranging beyond this year's limited talks on Iraqi security.
"I don't think you can overstate the importance of this," said the Republican senator Chuck Hagel. "If we're wise here, if we're careful, I think we have some opportunities."
As Manouchehr Mottaki, Iran's foreign minister, and other moderate conservative Iranian leaders publicly welcomed the US reappraisal today, the Senate majority leader, Harry Reid, urged the White House to mount a "diplomatic surge" to capitalise on the unexpected turnabout.
Stephen Hadley, Bush's national security adviser, seemed ready to embrace the idea, although he warned that it took two to tango.
"We need to keep the pressure up but also make clear there is a path for negotiation that will assess Iranian concerns," Hadley said. "But in the end of the day, the Iranians have to signal that they're willing to accept a negotiation path."
Belated US recognition that Iranian policy was being dictated by rational considerations of legitimate national and regional interest, rather than by the dangerous rantings of "mad mullahs", will also go down well in Tehran.
"Our assessment that Iran halted the [bomb-making] programme in 2003, primarily in response to international pressure, indicates Tehran's decisions are guided by a cost-benefit approach rather than a rush to a weapon irrespective of the political, economic and military costs," the NIE said.
"This in turn suggests that some combination of threats of intensified international scrutiny and pressures, along with opportunities for Iran to achieve its security, prestige and goals for regional influence in other ways, might - if perceived by Iran's leaders as credible - prompt Tehran to extend the current halt to its nuclear weapons programme."
All the same, this saga is far from over. Whether Mahmoud Ahmadinejad and fellow hardliners will bite on the proffered carrot is one of many imponderables arising from the US volte-face. Iran's president has played the "nuclear card" to telling effect in rallying domestic opinion, intimidating Arab neighbours, and forcing the west to take Iran seriously.
Nuclear ambiguity served Ahmadinejad well. He will not necessarily be happy that Washington now agrees he has no nuclear bomb to brandish. He will certainly ask why, if Washington believes there is no active weapons programme, it continues to punish Iran.
Hadley's insistence that the Iranian "threat" has by no means disappeared, and that the US must continue to pursue UN, allied and unilateral efforts to isolate Tehran diplomatically and economically, underlines this need for caution.
On a purely personal level, there is little reason to believe that the famously stubborn Bush and his hawkish vice-president, Dick Cheney, will suddenly admit they were wrong, swallow their enmity and make nice with Tehran.
Summing up their view, Hadley said the NIE "confirms that we were right to be worried about Iran seeking to develop nuclear weapons. On the other hand, it tells us that we have made some progress in trying to ensure that that does not happen. But it also tells us that the risk of Iran acquiring a nuclear weapon remains a very serious problem."
An affronted Israel, which believes it is in Iran's sights, certainly holds the latter view. The defence minister, Ehud Barak, flatly rejected the US findings yesterday: "It's apparently true that in 2003 Iran stopped pursuing its military nuclear programme for a time. But in our opinion, since then it has apparently continued that programme."
All the same, Bush will now struggle to make his case for additional international punitive measures against Iran. Russia, China and others, such as Germany, may use the NIE to urge negotiations. The International Atomic Energy Agency, led by the much criticised Mohamed ElBaradei, will rightly feel vindicated in its careful prior judgments.
And Democratic party presidential candidates will be encouraged in their arguments that Bush and Cheney were exaggerating the problem and spoiling for a fight. Hillary Clinton has been quick to make the point.
The story inside this story will obsess Washington for weeks to come. The extraordinary way in which the NIE was openly published (rather than partially leaked or kept secret, as would normally be the case) took everybody unawares - and possibly Bush, too, who was only told of its final conclusions last week.
On the face of it, the decision to go public looks like a case of high Washington politics - a pre-emptive strike against the White House by intelligence agencies and military chiefs determined not to be suckered, as they were before the Iraq war, into producing intelligence to fit a preordained policy.
That the CIA and others felt able to act in this manner is a measure of Bush's weakness and their own lingering anger over the Iraq WMD debacle. Yet for all that, the spooks admit they are still unsure of Iran's intentions. The irony here, amusing if it were not so deadly serious, is that having been badly wrong about Iraq, a chastened intelligence community, erring on the side of caution, may also be wrong about Iran.
Simon Tisdall