Book excerpt: America's power is in decline, its allies alienated, its soldiers trapped in a war that even generals regard as unwinnable. What has happened these past few years is well known. Why it happened continues to puzzle. Celebrated Slate columnist Fred Kaplan explains the grave misconceptions that enabled George W. Bush and his aides to get so far off track, and traces the genesis and evolution of these ideas from the era of Nixon through Reagan to the present day.