Of all the possible outcomes of the Afghan presidential election in August, the one that actually emerged – with supporters of the apparent victor President Hamid Karzai accused of widespread voting fraud – is perhaps the worst.

The final result was due to be announced last month but now that must wait on the results of an investigation by the United Nations-backed Electoral Complaints Commission. If enough fraudulent ballots are identified, there would be a powerful case for a run-off election between Mr Karzai and his nearest challenger, Dr Abdullah Abdullah.

This poses a dilemma for Afghanistan's Western sponsors. If they were to ignore the evidence of fraud and support a second term for President Karzai, it would leave them open to accusations of...