It was, as a senior European official said on Wednesday, “a courageous and substantial gesture” for the US to reverse its long-standing position and send William Burns, the state department’s third ranking official, to talks this weekend with Iran over its disputed nuclear programme.

The world has long known that the US is ready to use sticks in its efforts to stop Iran from nuclear enrichment. But Mr Burns’ presence with his counterparts in Geneva shows Washington supports the generous carrots being offered to encourage Tehran to stop the programme. The ball is now very firmly in the Iranian court. The package of international economic support, security guarantees and help with its civilian nuclear programme now on offer is one that Tehran should embrace.

The danger is that again Iran will try to play for time. It has used the past six months to increase the efficiency of its 3,000 spinning centrifuges. Another six months’ delay could allow Iran to further refine its enrichment capabilities to the point where it has all but mastered the technology. By then, it may reason, there could be a new, more sympathetic, occupant of the White House.

That would be a gross error. With offers of international help for its civilian nuclear programme, the benefits of continuing with industrial scale uranium enrichment would be minimal. Yet the economic costs are high and increasing almost by the week.

In the midst of an oil boom, Iran’s people are no better off, and may well be poorer, than they were 30 years ago. International sanctions against Iranian banks are tightening and world-class companies, the latest of them Total, are shunning the country as too unstable for investment.

Moreover, there is no doubt that Washington has more ideas up its sleeve if the current initiative fails, including further curbs on financial transactions and investment and a bar on insuring Iranian shipping. And there is still the chance that failure could trigger military action by Israel, a likely disaster for regional stability and the world economy, but no less damaging for Iran itself.

There is an opportunity here for Iran to grasp. It can continue to pursue its nuclear enrichment programme against the will of the international community and suffer further economic and political isolation, with possible costs for the regime’s own stability. Or it could embrace a settlement that offers economic opportunity and the prospect that Iran can begin to assume its rightful position among the community of nations.