"If people do not accept Hammond as the most important person from the Civil War, they have no understanding of the war" is an example of extreme either-or fallacy.
This type of fallacy puts forward an argument in a black-and-white fashion: the author presents a situation as if there were only two possible choices or outcomes, without considering that there may be several others.
Here, the author is saying that you either "accept Hammond as the most important person from the Civil War," or you don't understand anything about the war, which is an over-simplification.