This week, barring a last-minute climbdown, Iran may get back to building a nuclear bomb. It is a small moment, and a big one. Small because the threat has lingered for years; big because the consequences could convulse the region and the world.

If Iran ends its 30-month freeze on uranium tests, the long diplomatic mission by the West will be in ruins. The British Foreign Office says all bets will be off; the US secretary of state, Condoleezza Rice, signals that Iran is heading for the UN security council, and thus for resolutions and sanctions. Every diplomat and onlooker knows the steps of that quadrille. They danced it for Iraq.

As Iran moves towards the ultimate in WMD, George Bush must be thinking he fought the wrong war. Now, as Israel says Iran's nuclear missile programme "can be destroyed", the scent of another conflict hangs in the air. Even if President Mahmoud Ahmadinejad steps back from the brink, as he is prone to, there is a wider threat.

In the past week a 55-page European intelligence assessment has surfaced. It is an audit of the quest by rogue states to buy the kit to make weapons of mass destruction. Syria and North Korea have been stocking up, along with Pakistan, and Iran is allegedly working towards a long-range missile that could reach Italy.

In addition, the New York Times reporter James Risen claims in a new book that the CIA inadvertently helped Iran to build a nuclear bomb by supplying flawed blueprints that the country's scientists may have corrected and used. Obviously, leaked intelligence on WMD comes with some health warnings. How dodgy, exactly, are these dossiers?

Maybe the CIA is daft enough to offer up DIY bomb manuals, though this sounds implausible. The Tesco-isation of the nuclear trade raises another caveat. According to the leaked European report, the world is a shopping mall for nukes, and the boardrooms and universities of Europe are the bombers' Bond Street.

Hardly any of this is top secret or new. Dubious regimes have indeed been stockpiling illicit technology, as if buying beans at a supermarket. A briefing for the Carnegie Endowment thinktank, published in September 2005, two months after the leaked report was written, lists a worldwide history of nuclear deals, including Iran's links with the rogue Pakistani scientist, Abdul Qadeer Khan.

Nuclear arms never held many surprises. J Robert Oppenheimer, their inventor, called himself "the destroyer of worlds"; hopes that lavish death could forge a better universe faded faster than the prayers of the pastor who commended to God the Enola Gay, bound for Hiroshima. Sixty years on, the notion of nuclear nemesis has not sunk in. Last year's make-or-break US conference to revive the nuclear non-proliferation treaty achieved nothing.

The pact, ratified in 1970 and signed by 187 countries, was designed to ensure that unarmed states never acquired weapons and that armed nations, in return, would wind down their arsenals.

That cornerstone of a peaceful world is crumbling, partly because Bush wants new weapons while demanding that other regimes forswear them, but also because the treaty is fatally flawed. Its aims, to eradicate nuclear weapons while championing the spread of nuclear energy, are irreconcilable. Atoms for Peace, suspect in President Eisenhower's day, is an oxymoron in a globalised age.

Any rogue state can build up a civil programme, opt out of the treaty with six months' notice and begin making weapons. Iran has always claimed, to universal disbelief, that it is only exercising its right to enrich uranium for peaceful purposes. Pakistan, a non-signatory, was last week reported to be buying up to eight reactors from China, which has long been suspected of helping with its weapons programme.

On the campuses of Tehran, even moderately minded students are aggrieved. Who are Bush and Tony Blair to preach while laying in new nukes and welcoming India, with its illicit weapons, into their nuclear club? Israel is stacked with unauthorised nukes, a Nato base sits at Herat, and the US Fifth Fleet patrols the Persian Gulf. Why should Iran, so besieged, not have a deterrent?

One answer is that Ahmadinejad is a Holocaust-denier who thinks that Israel should be "wiped off the map". No one could sleep easy with his finger trembling on a nuclear button. But there are more dangerous prospects even than the Iran bomb. Professor Paul Rogers, of Bradford University, thinks it conceivable that Israel could launch a pre-emptive attack against its loose-mouthed neighbour. Ariel Sharon's successor, needing to look tough, may be keener than his predecessor to do so.

Israel would have to move fast, though. A few months from now, Iran's powerful Bushehr reactor could be up and running and few attackers would dare unleash a reprise of Chernobyl.

Such a threat may seem far-fetched, but Bush's tolerance for a regime he calls "the world's primary sponsor of terror" is as thin as skin.

What a year for Britain to announce a $44bn replacement for the Trident missile, plus a new generation of nuclear power stations. Tony Blair, intent on both schemes, is braced for protests on weapons but foresees little trouble over energy.

Britain is not keen on taking fewer cheap flights and turning out the bathroom light. Nuclear power, touted as a painless option, is costly, risky and produces waste that stays dangerous for 240,000 years. A combination of renewables, energy saving, fossil fuels and carbon storage could provide an alternative. Besides, Britain has a duty to set some example. On nuclear power, the case is far from made.

Trident, by contrast, is straightforward. Its replacement should be fought by everyone bothered about world security. More weapons for Britain would be a come-on to every failed state on the planet.

In Israel a leader lies in hospital. In Iraq 130 people were blown up on Thursday last week. In North Korea promises of nuclear disarmament have withered. In Iran the nuclear scientists resume their work. This may be the most dangerous time since superpowers threatened mutually assured destruction.

The world, once an atomic Athenaeum, has become a bomber's eBay, full of murky bidders with pseudonyms and of nukes for auction. The nuclear aristocracy is dead, destruction is democratised and Iran, angry at the West's hypocrisy on the nuclear race, believes that what's good for London is good for Tehran.

There is also hope. Iran's theocrats might oust Ahmadinejad. American voters may elect a president who realises, as Blair must too, that the West has to reduce its own arms if it is to persuade others to do likewise. But, for now, cold war politics looks easy. The abyss was always safer than the quicksand. This time, we cannot simply walk away.

Illustration: Matt Kenyon