George W. Bush has been all but untouchable in the war on terror, and he has the poll ratings to prove it. Now, for the first time, doubts are surfacing publicly in Washington—and knives are being sharpened—over what Bush knew about the threat from Osama bin Laden and when he knew it. Most of the questions center on a recently disclosed intelligence briefing on Aug. 6, 2001, at which the president was warned that, among other threats, Al Qaeda-linked terrorists might try to hijack an airliner. Considering that, at about the same time, FBI agents in Phoenix and Minneapolis...