By pursuing this approach, the Administration would find itself in a win-win situation: if engagement succeeds, Obama could turn the page in US-Iran relations and herald a new Middle Eastern order. It would be a genuine triumph of diplomacy and a great early success for the new presidency. If engagement fails, Obama could argue that he had tried in earnest and failed to budge Tehran. The time would have come for tougher measures to thwart Iran's nuclear ambitions and a US-led effort would encounter much less resistance after diplomacy had been tried. The model would be Iraq - not 2003, but 1991.

The theory makes perfect sense, except for one small detail: the time factor. The French foreign minister, Bernard Kouchner, recently gave a clear warning to the new president - negotiate with Iran at your own peril. Kouchner delicately demolished the notion that dialogue had not been tried - the Europeans, after all, had engaged Tehran for more than six years and had received nothing in return. All that diplomacy achieved was for Iran to push back European red lines while making no real concessions. What guarantees will Obama receive that US engagement will not fall into the same trap of a long-drawn-out diplomatic dance, while Iran gains time to acquire nuclear weapons? The answer is that dialogue will occur under a self-imposed deadline, after which the chips are down.

But mid-2010 might already be too late for Obama to switch gears. Unlike the negotiating clock, which is always ticking and ticking slowly, Iran's nuclear clock is ticking very fast, and is getting faster every day. Less than two weeks after Obama was elected, a new International Atomic Energy Agency report offered conclusive evidence that Iran has accumulated enough low enriched uranium that, if reprocessed, could yield enough high enriched uranium for one rudimentary nuclear bomb. There is no way of telling if Iran can reprocess and enrich to that level. According to the same report, Iranian centrifuges are enriching only 4.9 per cent, whereas they would have to enrich up to 80 or 90 per cent to produce weapons-grade uranium. But Iran is installing new-generation centrifuges and its installations are improving their performance all the time. Experts at the Wisconsin Project for Arms Control suggest that Iran could have enough fissile material for one bomb by Inauguration Day and another by Easter 2009.