It was a telephone call three days ago from Hillary Clinton that tipped the scales. The US Secretary of State called President Karzai to reassure him of America’s support if he agreed to a run-off. After two months of intense talks between Washington and Kabul, the Afghan leader finally relented.

The diplomacy leading to the decision to hold a run-off began the day after the disputed August 20 vote. Washington started piling pressure on Mr Karzai as soon as allegations of fraud surfaced. The first task, one official said, was to persuade the Afghan leader to “keep quiet”, not to declare victory, and wait for the result of the election review. That crucial objective was met.

There were regular calls between US officials and Mr Karzai over the past two months, while Karl Eikenberry, the US Ambassador to Afghanistan, played a major role on the ground.

The day after the August election, Richard Holbrooke, Mr Obama’s special envoy for Afghanistan and Pakistan, had a heated conversation with Mr Karzai, in which he urged him to agree to a second-round vote.

Since then Mr Holbrooke has barely been seen. Officials insist that he has been involved behind the scenes but there is little doubt that, given the delicacy of the negotiations, Mr Holbrooke’s role was reduced. Mrs Clinton, meanwhile, took on a far more active part, as did John Kerry, who is chairman of the Senate Foreign Relations Committee. Mr Kerry extended a visit to Kabul to negotiate the final run-off deal.

America’s closest allies also weighed in. Gordon Brown has had four conversations in a week with Mr Karzai. Diplomats said that when the Prime Minister spoke to Mr Karzai on Monday he was still wavering. Mr Brown told him that he would win international respect if he allowed the process to be completed.

There remain myriad problems. The Obama Administration hopes a run-off election will lay the groundwork for a unity government with Mr Karzai’s opponent, Abdullah Abdullah — but that is far from certain. It must persuade the candidates not to cheat in this election. If Mr Karzai clings to power, he must immediately begin staffing his government with reformers, not drug barons, cronies and warlords.

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