The appearance of extensive fraud in Afghanistan’s recent presidential election threatens the Obama administration’s efforts to help Afghans escape from a 30-year nightmare of warfare, warlordism, and corrupt misrule. There are pitfalls to any position President Obama takes on the “clear and convincing evidence of fraud’’ that a United Nations-backed Electoral Complaints Commission has found. There is a danger in being identified with a reelected President Hamid Karzai who lacks legitimacy - and a different kind of risk in acting like a colonial power that discards its former favorite at its own convenience.

The principle guiding US policy ought to be respect for the democratic rules of the game - as opposed to backstage support either for Karzai...