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.