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Massive fraud in the first round of Afghanistan's presidential elections, held Aug. 20, plunged the country into a seven-week political crisis that gave the Taliban its greatest strategic gains in eight years of war. As things now stand, the second round, scheduled for Saturday, will be even worse.

The runoff election faces profound challenges. Afghanistan's fraudulent first round undermined President Hamid Karzai's credibility both at home and in key countries contributing troops to the region. Electoral misconduct is one important reason that President Obama is rightly reconsidering the wisdom of sending an additional 40,000 troops. The Taliban, which benefited from chaos created by the August fraud, now has every incentive to wreak havoc in the second round. Voters, many...