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If good intentions ever paved a road to hell, they are doing so in Afghanistan. History rarely declares when folly turns to ­disaster, but it does so now. Barack Obama and his amanuensis, Gordon Brown, are uncannily repeating the route taken by American leaders in Vietnam from 1963 to 1975. Galbraith once said that the best thing about the Great Depression was that it warned against another. Does the same apply to Vietnam?

Vietnam began with Kennedy's noble 1963 intervention, to keep the communist menace at bay and thus make the world safe for democracy. That is what George Bush and Tony Blair said of ­terrorism and Afghanistan. Vietnam escalated as the Diem regime in Saigon failed to contain Vietcong aggression and was deposed with American ­collusion. By 1965,...