Scholars, who pride themselves on speaking their minds, often engage in a form of self-censorship which is called "realism." To be "realistic" in dealing with a problem is to work only among the alternatives which the most powerful in society put forth. It is as if we are all confined to a, b, c, or d in the multiple choice test, when we know there is another possible answer. American society, although it has more freedom of expression than most societies in the world, thus sets limits beyond which respectable people are not supposed to think or speak. So far, too much of the debate on Vietnam has observed these limits.
Howard Zinn on War (2000), Ch. 14: Vietnam: A Matter of Perspective