Moving to avoid war in Iran in the final months of his administration, George Bush has approved the highest-level American diplomatic contact with its ideological enemy since the humiliating US embassy hostage crisis of 1979.
Shifting from bellicose threats to diplomacy, Mr Bush is sending an envoy to talks this weekend aimed at curbing Iran's nuclear ambitions. The rapprochement includes plans by the US to post diplomats in Tehran for the first time since the Islamic revolution in the form of a US interests section – a move halfway to setting up an embassy – subject to approval by the Iranian President, Mahmoud Ahmadinejad.
There was consternation in Washington last night when details were leaked that the US is edging towards diplomatic recognition raising fears that the weekend's talks could be cancelled by either side.
Iran has complained it is unable to deal directly with the US. Until now, the US has demanded that Iran halt its nuclear enrichment programme as a precondition for any serious diplomatic contact.
William Burns, the third in line at the State Department, will travel with the European Union's foreign policy chief, Javier Solana, to Geneva to talk to Iran's main nuclear negotiator, Saeed Jalili, as the US tries to halt a dangerous slide towards conflict, diplomatic sources said. US officials described the decision to send Mr Burns as "a one-time deal". But there is little doubting Washington's desire to try to break the impasse over Iran's nuclear programme without going to war.
The diplomatic overture follows unequivocal advice to the US that a military strike on Iran would not change the country's nuclear policy. The Rand Corporation, a leading consultant, said in a report for the US Air Force that any attack on Iran's nuclear facilities would only bolster the regime of Mr Ahmadinejad.
"If Iran's facilities were to be bombed, public support for any retaliation its government took would likely be widespread," its report said. "Attacks on Iran proper would generate a great deal of ill-will and, in our view, would be unlikely to change Iranian policy." Tehran test-fired some of its long-range ballistic missiles last week in a signal that it is taking the threat of an attack by Israel or the US seriously.
Barack Obama, the Democratic presidential candidate, has called current policy barring face-to-face contacts with Iran as counter-productive, and is supported in his view by a majority of Americans, according to opinion polls. Mr Obama says one of his first steps as president would be to end the ban on US officials talking to Iran.
By resorting to talks with North Korea, rather than bellicose sword-waving, the US can claim to have reaped rewards recently in persuading it to end its nuclear-enrichment programme.
Ivo Daalder, a senior fellow at the Brookings Institution, a think-tank in Washington, said: "I'm glad the administration has finally decided the only way to get a peaceful resolution of the nuclear issue is actually to talk to them. But it will take a sustained and serious engagement, rather than a one-time meeting, to see whether an acceptable deal can be done."
But US officials were adamant that Mr Burns will not be permitted to negotiate directly with the Iranians or to hold separate bilateral meetings. His only role, they said, is to press the White House position that Iran must suspend its enrichment of uranium before direct talks can begin.
Diplomacy between the US and Iran has been a political minefield since the hostage crisis of 1979 when students stormed the US compound in the capital. For the next 444 days, 52 US diplomats were held against their will in what became a national ordeal for Americans. It culminated in a botched rescue mission that killed eight American servicemen.
The decision to send a high-level official to the next round of talks follows months of recriminations between Washington and Tehran. At the Geneva meeting, Iran is expected to respond formally to proposals that Britain, China, France, Russia, Germany and the US put forward in June.
Only a month ago, a State Department spokesman, Sean McCormack, said the US would boycott the meetings unless "Iran suddenly has a change of tune".
Senior European diplomats were described as ecstatic at the overnight breakthrough. They believe it will send a message to Iran that the international community wants a negotiated rather than a military solution to the standoff.
To encourage a climbdown, Mr Solana is proposing a six-week "freeze for freeze". In this period, Iran would not speed up nuclear enrichment and the US would not push for more sanctions.
In June 2006, the six-nation talks offered to bring the country in from the cold, providing it halted nuclear enrichment. That offer was rejected and the programme was speeded up. Last June, the six sweetened the offer in a new deal. A response from Iran led to the Burns decision.