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From Geneva yesterday come all kinds of good diplomatic vibrations. Iran may allow U.N. inspectors into a recently unveiled uranium-enrichment plant “within two weeks.” Another meeting will be held before month’s end. A “freeze” on sanctions was bruited about. In an appearance at the White House, President Obama sounded sober but hopeful, calling the direct American talks with the Islamic Republic “a constructive beginning” toward “serious and meaningful engagement.”

Mahmoud Ahmadinejad was presumably in even better spirits at his remarkable change of fortune. A month ago, Iran’s president was struggling to cement his grip on power after stealing an election and repressing nationwide protests. A week ago, the disclosure of the secret facility near Qom highlighted Iran’s chronic prevarication and raised calls for more sanctions.

By yesterday, all that had changed. At the 18th-century Villa Le Saugy, Iran’s representative sat among the world’s powers as a respected equal. Responding to an overture from the Obama Administration, the Iranians even talked about the future of the U.N. and other nonnuclear issues. Meanwhile, Washington was “buzzing” (as one newspaper put it) that a one-day visit by Iran’s foreign minister might signal more detente to come. Back in Tehran, Mr. Ahmadinejad floated a tete-a-tete with the U.S. President. In short, this engagement conferred a respectability on his regime that Mr. Ahmadinejad could only have imagined amid his vicious post-election crackdown.

The price of entry is surprisingly modest, too. Though cautious, the P5+1 (the veto-wielding Security Council members, plus Germany) welcomed signs of Iranian concessions: Inspectors at Qom, an openness to send low-enriched uranium outside Iran for enrichment, possibly suspending its own enrichment program. Mr. Ahmadinejad said the Geneva talks were “a unique opportunity” for the West.

Consider the Iranian offers in turn. Inspectors from the International Atomic Energy Agency won’t find anything incriminating at the Qom facility. Having lied about it for years, the Iranians now have plenty of time to clean the place out. Iran’s experience with the IAEA goes back to the first inspections starting in 1992, which somehow prevented the world from learning about Iran’s bomb program for a decade and then only from an Iranian dissident group. A freeze on enrichment used to be the U.S. precondition for talks with Iran. Now the U.S. and Europeans say that in exchange merely for this enrichment promise, they’ll freeze any additional sanctions.

Iran has timed its olive branch well. The Europeans are more frustrated with past Iranian stalling than is Washington and have started to hanker for tougher measures. Those demands will now be muted. For years, Iran has talked with the Europeans, using the time and diplomatic cover to make nuclear progress. The Obama ascendency offers the mullahs another chance, with an even more eager partner, to repeat the exercise with a far bigger potential payoff. Expect Iran to follow the North Korean model, stringing the West along, lying and wheedling, striking deals only to reneg and start over. In the end, North Korea tested a nuclear device.

On long evidence, the regime has no intention of stopping a nuclear program that would give it new power in the region, and new leverage against America. The Qom news reveals a more extensive, sophisticated and covert nuclear complex than many people, including the CIA, were willing to recognize. The facility is located on a Revolutionary Guard base, partly hidden underground and protected by air-defense missiles. Its capacity of 3,000 centrifuges is too small for civilian use but not for a weapons program. It’s a good bet an archipelago of such small covert facilities is scattered around Iran.

Meanwhile, news reports this week say German and British intelligence believe Iran never stopped clandestine efforts to design a nuclear warhead. Their assumptions contradict the U.S. National Intelligence Estimate in 2007 that Iran had stopped its nuclear weapons program in 2003 and kept it frozen.

The evidence is overwhelming that the window to stop the world’s leading sponsor of terrorism from acquiring a bomb is closing fast. If we are serious about doing so, the proper model isn’t North Korea, but Libya. The Gadhafi regime agreed to disarm after the fall of Saddam Hussein convinced its leaders that their survival was better assured without nuclear weapons. Mr. Ahmadinejad and Iran’s mullahs will only concede if they see their future the same way.

This supposed fresh start in Geneva only gives them new legitimacy, and new hope that they can have their bomb and enhanced global standing too.