[andyfox]

Your points are good ones, but David's point is a more limited one. If I understand it correctly, it is that if we fought the war to prevent Vietnam from becoming a Communist gulag, and we lost the war and the Communists took over, and it didn't become anything close to the disaster we said it would, then that it compelling evidence--not proof--that fighting the war was failure from a cost-benefit analysis perspective. It my well be that a no-war Vietnam would have been different than post-war Vietnam and that David's formula is therefore not "proof." But he is not claiming it is proof; just a compelling piece of evidence.