Paul Pillar, a former CIA agent, says that good intelligence work doesn’t matter all that much to presidents on big decisions, which they make according to their ideological instincts. Excerpt:

 From George W. Bush trumpeting WMD reports about Iraq to this year’s Republican presidential candidates vowing to set policy in Afghanistan based on the dictates of the intelligence community, Americans often get the sense that their leaders’ hands are guided abroad by their all-knowing spying apparatus. After all, the United States spends about $80 billion on intelligence each year, which provides a flood of important guidance every week on matters ranging from hunting terrorists to countering China’s growing military capabilities. This analysis informs policymakers’ day-to-day decision-making and sometimes gets them to look more closely at problems, such as the rising threat from al Qaeda in the late 1990s, than they otherwise would.

On major foreign-policy decisions, however, whether going to war or broadly rethinking U.S. strategy in the Arab world (as President Barack Obama is likely doing now), intelligence is not the decisive factor. The influences that really matter are the ones that leaders bring with them into office: their own strategic sense, the lessons they have drawn from history or personal experience, the imperatives of domestic politics, and their own neuroses. A memo or briefing emanating from some unfamiliar corner of the bureaucracy hardly stands a chance.

Besides, one should never underestimate the influence of conventional wisdom.

Neuroscientist Michael Gazzaniga says that the brain has a mechanism that makes decisions prior to the subject becoming aware that the decision has been made. This does not mean that one doesn’t have free will; it means that all the decisions and experiences leading up to the point of decision-making have conditioned this unconscious apparatus. Sounds to me like the neuroscientific reason behind Paul Pillar’s observation above. So often we think we’re reasoning, but in fact we’re rationalizing a decision we’ve already made, whether or not we’re consciously aware that we’ve made it.