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Iran's Interior Ministry announced Saturday that incumbent President Mahmoud Ahmadinejad had won 63.29% of the vote in the country's presidential election — a landslide. But Iran's opposition leader Mir-Hossein Mousavi says he won and that the result had been rigged; Mousavi supporters have taken to the streets in Tehran and other cities to protest the official outcome.

Ahmadinejad, for his part, insists that he won fairly, while Iran's Supreme Leader, Ayatullah Ali Khamenei, initially congratulated Iranians for proving their "great worth." The result, Khamenei said, was a "divine assessment." On Monday, though, Khamenei ordered the powerful Guardian Council to investigate the fraud allegations.

So far, Washington has taken a cautious approach to...