An important skill of critical reading is the ability to detect an author's bias and prejudice. The reason you need to be able to do this, of course, is that bias and prejudice may invalidate an author's claim.
There are several ways to detect an author's possible bias and prejudice, for example:
Bias and prejudice may be the result of national pride and chauvinism (as may be the case for Japanese scholars' denial of the Rape of Nanking) or personal or professional rivalry (as in the Browning-Goldhagen controversy; on both of these issues, see Evaluating Contradictory Data and Claims); perhaps an author's bias and prejudice is a result of a specific agenda he or she wishes to support (see Historiography); or perhaps it simply reflects the author's ignorance and resulting ignorant worldview. When writing about history, our ability to detect and identify such bias and prejudice is a valuable and necessary skill.
Please note that bias and prejudice, although they may invalidate an author's claims, do not necessarily disqualify that author's work as a suitable subject for consideration. If you do include references to biased and prejudiced works, however, be sure to qualify your references by pointing to the works' underlying bias.
Note that even secondary sources written by reputable authors are not immune to bias and prejudice. Be sure to carefully examine any author's premise, argument, and methodology to ascertain whether he or she has consciously or unwittingly pursued the above strategies.
You are even more likely to encounter prejudice and bias in primary sources: newspaper editorials, political cartoons, and "private" sources such as letters, diaries, etc. are often blithely biased, as they are generally intended for a specific audience that is assumed to share the author's view. (On this, see also fiction.)
Finally, beware of presenting biased or prejudiced views of your own, or manufacturing dishonest evidence. When writing, seek to avoid the pitfalls of the four negative strategies described above - on which, see also Don'ts and The Ethics of Quoting.
We conclude this section with an obvious example of bias and prejudice - one that again falls within the parameters of our larger theme of the events leading up to World War II - from a well-known source: Adolf Hitler's Mein Kampf.
Hitler's Mein Kampf (1924) was written during his brief period of incarceration following the failure of the beer hall putsch, his abortive effort in 1923 to overthrow the post-World War I German government. The book amounts to a detailed manifesto in which Hitler articulates his theories of history, politics, nationhood, and race. Historians recognize in the book, written ten years before Hitler assumed control of Germany, a blueprint of the basic strategies, both domestic and foreign, he was later to pursue. The following excerpts refer to his views on nature and racial mixing, specifically, his views on the possible union of non-Jewish Germans ("Aryans") with Jews (whom Hitler routinely refers to as "a pack of wolves," a "herd of rats" and "parasite[s]" [301, 302, 304]):
Hitler on the "natural" order of things:
Even the most superficial observation shows that Nature's restricted form of propagation and increase is an almost rigid basic law of all the innumerable forms of expression of her vital urge. Every animal mates only with a member of the same species. The titmouse [a bird] seeks the titmouse, the finch the finch, the stork the stork, the dormouse the dormouse, the wolf the she-wolf, etc. Only unusual circumstances can change this [Hitler is evidently thinking of the outcome of a union between a donkey and a horse, whose product is a mule], ... [b]ut then Nature begins to resist this with all possible means, and her most visible protest consists either in refusing further capacity for propagation to bastards [mules cannot procreate] or in limiting the fertility of later offspring; in most cases, however, she takes away the power of resistance to disease or hostile attacks ... the offspring will probably stand higher than the racially lower parent, but not as high as the higher one. Consequently, it will later succumb in the struggle against the higher level. Such mating is contrary to the will of Nature (284).
Later, applying the above logic to humans:
With satanic joy in his face, the black-haired Jewish youth lurks in wait of the unsuspecting girl whom he defiles with his blood, thus stealing her from her people. With every means he tries to destroy the racial foundations of the people he has set out to subjugate ... [with] the clear aim of ruining the hated white race by the necessarily resulting bastardization, throwing it down from its cultural and political height, and himself rising to be its master (325).
Notice that Hitler here is guilty of all four of the above-listed criteria identifying bias and prejudice:
While Hitler's is a particularly egregious example, your job as a student of history is to hone your critical reading skills and learn to recognize, deconstruct, and challenge the many variations of such bias and prejudice, whether it be subtle or, like Hitler's, blatant.