It doesn’t take a fevered brain to assume that if Iran’s ayatollahs get their hands on the bomb, the world could be in for some nasty surprises. Iran’s claim that its nuclear programme is entirely peaceful is widely disbelieved. That is why Russia and China joined America, Britain, France and Germany at the UN Security Council to try to stop Iran enriching uranium. Until two months ago they seemed ready to support a third and tougher sanctions resolution against Iran. But then America’s spies spoke out, and since then five painstaking years of diplomacy have abruptly unravelled.

The intelligence debacle over Iraq has made spies anxious about how their findings are used. That may be why they and the White House felt it right to admit, in a National Intelligence Estimate in December, that they now think Iran halted clandestine work on nuclear warheads five years ago. As it happens, this belief is not yet shared by Israel or some of America’s European allies, who see the same data. But no matter: the headline was enough to pull the rug from under the diplomacy. In Berlin last month, the Russians and Chinese made it clear that if there is a third resolution, it will be a mild slap on the wrist, not another turn of the economic screw.

At the same time, Iran is finding an ally in the International Atomic Energy Agency. Its director-general, Mohamed ElBaradei, is a Nobel peace-prize winner who is crusading to confound those he calls “the crazies” in Washington by helping Iran to set its nuclear house in order, receive a clean bill of health and so avert the possibility of another disastrous war...

Yet Iran’s argument is a travesty. Although the National Intelligence Estimate does say that Iran probably stopped work on a nuclear warhead in 2003, it also says that Iran was indeed doing such work until then, and nobody knows how far it got. The UN sanctions are anyway aimed not at any warhead Iran may or may not be building in secret, but at what it is doing in full daylight, in defiance of UN resolutions, to enrich uranium and produce plutonium. We need this for electricity, says Iran. But it could fuel a bomb. And once a country can produce such fuel, putting it in a warhead is relatively easy.

Excerpted from a leader in the February 2 issue of ‘The Economist’