Which kind of intelligence failure is better — the kind that badly understates a threat, such as the one in London, or the kind that overstates a threat, such as the insistent warnings before the invasion of Iraq that Saddam Hussein was armed with weapons of mass destruction? Even in the best intelligence services, failures are sometimes inevitable. Foresight will never be as sharp as hindsight. Only after the fact — after the Underground blows up, after 9/11, after the stockpiles of biological and chemical weapons are nowhere to be found — is it clear what the picture looks like...