The theory is that if we learn about it we can stop it from repeating itself. In my opinion the theory is wrong history will repeat itself whether we read about it or not. There is always a beginning of a country a peak and a downfall and that is read about for just about every country that made it to the history books. There is always a tyrant who will lead a peoples into war over something. There is always a person that doesn't like a certain type of people for a reason and gets enough power to harm the population of those. For example Isis is another one of these. Like Hitler they target specific group of peoples and try and wipe them out.
Research, however, suggests that reading fiction may provide far more important benefits than nonfiction. For example, reading fiction predicts increased social acuity and a sharper ability to comprehend other people's motivations.
