Rarely, in the 30 years of name-calling and visceral animosity between the US and Iran triggered by the 1979 Islamic Revolution, has the prospect of detente between them been so tantalisingly within reach. But Tehran is recklessly close to frittering away this opening. That would be a disaster.

The tentative deal struck at landmark talks in Geneva at the beginning of this month bought vital time to stave off what was looking like an inexorable confrontation between Iran and the west over Tehran’s nuclear ambitions.