What was a common belief among historians in the early 20th century that makes their sources biased?

That entire groups of people can easily be classified by stereotypical traits.

Explanation:

This was a common belief among historians in the early 20th century. This behaviour was particularly noticeable when historians dealt with cultures outside of the Western canon or people who were not Caucasian. Because many historians believed this broad characterizations to be accurate, they often did not take the time to get to know individuals more deeply, which means that their findings were biased due to their prejudice.

what are the options?

no because we were not ready for war