As a young theorist, Ellsberg had noticed that it's hard to get people to change a course they've set based on bad information, even after you've given them better information. But in the case of Vietnam (and, by extension, Iraq) there is another explanation for the failure of accurate information to produce a single, rational outcome: the decision-makers are making value judgments about how important the goal is and how high a price they are willing to pay to achieve it. American Vietnam policy mystified and enraged Ellsberg because its goal, preventing Vietnam from becoming a Communist-governed country, was much less valuable to him than it was to Congress, the public, or the various Presidents during the years when the American commitment was being ratcheted up to the level of full-scale war.

Why did Johnson and Nixon stick with a war that was going so poorly? Partly, it was a matter of wanting to avoid humiliation once the United States committed its forces. Also, they had an alternative scenario in mind: a Vietnam that looked like Korea, with a tense but stable relationship between a Communist North and a pro-American South. South Vietnam might even have moved from puppet governance to democracy, as South Korea has. And Johnson, at least, rejected an even more hawkish policy - using military force more aggressively in the hope of forcing the North to surrender. Ellsberg says that such a policy inevitably would have led to the use of nuclear weapons and a war with China, but Johnson's policy, combined with the inexhaustible determination of the North Vietnamese, produced only protracted stalemate. Johnson was balancing public and congressional opinion, the goal of preventing South Vietnam from becoming Communist, his chances of success, and the likely cost in blood and treasure of the course he chose. If he had been willing to pay any price, he might have won; if he hadn't cared about the goal, he wouldn't have escalated the war. Instead, he wound up with a compromise policy that failed horribly, and a great deal of death and destruction occurred to no end. Presidents Truman, Eisenhower, and Kennedy made similar illogical-seeming decisions, as Ellsberg discovered when he read the Pentagon Papers, not because they were ill-informed but because they, too, regarded preventing Communist rule in South Vietnam as extremely important, though not important enough to demand all the military options at their disposal.

In the end, the Vietnam War can't be reduced to a problem of miscalculated probability. It is of the utmost importance right now that we understand that the decision to go to war is ideological, not informational: the reason people disagree vehemently about war in Iraq is not that the facts on the ground or the true prospects of American military success are being kept hidden. What they disagree about is under what conditions and by what means the United States should try to affect the governance of other countries. It's not what we know but what we believe in that makes all the difference.