Leading article: Diplomacy, not war, is the way to confront Iran

This week's report by United Nations inspectors on Iran's nuclear programme banishes what little doubt remained that the Islamic Republic is seeking a nuclear weapons capability. The International Atomic Energy Agency is careful not to say that Tehran is currently building a weapon, or that it will start doing so in the near future. But it provides irrefutable evidence of computer models and work on detonation devices whose only conceivable use is to develop a nuclear warhead.

The report is the bluntest yet from a body that some have accused in the past of treating Iran with kid gloves. Four years ago, a top-level US intelligence assessment concluded that Iran had in fact stopped work on a nuclear weapon in 2003. But the IAEA believes that such activities have secretly been resumed by the regime. In doing so, it has only exposed the lack of options for the West as it attempts to stop them – a lack underscored by the muted reaction from the White House, confined thus far to formulaic reiterations that an Iranian nuclear weapon is "unacceptable", and that "all options are on the table", including the military one.

Inevitably, there have been calls for even tougher sanctions. But history shows that orthodox sanctions to bend a country's will rarely work. Some demand so-called "lethal" sanctions, such as a blockade on Iranian energy exports. But in the unlikely event that such a move was feasible, it would amount to an act of war. It would also have to be undertaken by a "coalition of the willing", given the Russian and Chinese vetoes that would surely block the adoption of new sanctions by the UN Security Council.

Meanwhile, political pressure on President Obama to "do something" only intensifies, especially after last month's allegations of a bizarre Iranian plot to assassinate Saudi Arabia's ambassador to Washington. Then there is the Israeli dimension. With the 2012 election approaching fast, Mr Obama cannot be seen to be "soft" on Iran, in other words as unsympathetic to Israel, when he is already losing support among America's Jewish voters.

There is no argument about the peril posed by a nuclear-armed Iran. It would constitute a threat to Israel, increase Tehran's powers of blackmail over its neighbours, and almost certainly set off a regional nuclear arms race, led by Saudi Arabia. But, as almost every wargaming study on the subject has made clear, the bombing of Iran's atomic installations would have no less devastating consequences.

Recent weeks have been abuzz with reports that Israel is on the verge of a pre-emptive strike, akin to those it launched against Iraq in 1981 and Syria in 2007. But such action would convulse the entire Middle East, while the fragile world economy would be dealt a massive blow, assuming Tehran retaliated by blocking the Straits of Hormuz, through which 40 per cent of world oil exports pass.

What is more, Iran has been planning for such an eventuality. Its nuclear sites are well protected; there is no guarantee that air strikes alone would be successful. To be certain, the West would have to invade and occupy Iran. And that, for the US and its allies, is simply a non-starter.

There is only one solution. Iran – a country that no more than any other appreciates being dictated to by outsiders – must somehow convince itself that its own best interests lie in not developing a weapon. That trick has been pulled off before, in the cases of Brazil, Argentina and apartheid-era South Africa. And an opening surely exists in the current tensions between the clerics and President Mahmoud Ahmadinejad, and the latter's unpopularity. That opening must be exploited, not by bombs, but by diplomacy.