While we often think of our memory as file cabinet and when we want to retrieve a memory we just go into the drawer and pull it out. Through experiments scientist have concluded that this file cabinet view is wrong and in actuality each time we remember something we create a new memory, so the more we think about something the less it is like what actually happened.
So what does this have to do with US history in film? Consider Birth of a Nation, a movie about slavery, Civil War, and Reconstruction. Knowing the majority of US citizens go to the theater at this point in history, while they watch the movie they are forced to think about the time period. In thinking about that memory, either from what was learned or actual memories from the time, they are recreating it. So their memory is being slightly skewed towards what the film has presented.
Historical films force us to think about history (duh) and with that the memories of what we have learned about or experience during the time period. Knowing visual and audio leave a lasting impression on us and that we create new memories every time we remember, watching films can change (without us even realizing it) the way we view historical events.