A month after the murder of four American officials in Libya on Sept. 11, congressional testimony and leaked government cables have revealed that some U.S. officials immediately recognized that terrorists had planned the attack. So why did the Obama administration's top policy makers—including the president himself—persist in claiming that the catastrophe was a spontaneous outburst of rage against an anti-Islam video posted on YouTube by an American provocateur?

Many critics smell cynical politics. The president, after all,...