Saturday’s talks in Istanbul between Iran and the five permanent members of the UN Security Council plus Germany are testimony to the power of diplomacy backed by the threat of force. Just as a creative diplomatic initiative is required to prevent a calamitous conflict, no one should doubt that Tehran is coming to the table reluctantly, and under extreme duress. Sanctions of unprecedented breadth and depth are raising the cost of Tehran’s defiance of the Security Council’s demands.

Barack Obama has brought the case for diplomacy into the open, speaking powerfully against a “rush to war”. At the same time, he has made clear the conditions in which war becomes inevitable. If Iran acquires a nuclear weapon, America will strike. This clarity about red lines and the consequences if they are breached has been sorely missing. What matters now is that negotiations are seized with purpose and imagination, and are not allowed to falter at the first sign of difficulty.

We know that Iran will be engaging with the world at a time of turbulence and change within the regime. In a striking sign that Ayatollah Khamenei and those around him are forgetting the lesson of the Shah (and, much later, Hosni Mubarak in Egypt), the supreme leader is asserting power as the main pillar of the state, while rival centres of influence are weakened. In the long term, this will make the regime more vulnerable to change from below. In the immediate future it may make Iran a more disciplined negotiating partner.