According to eric foner, why is freedom an effective theme to focus on in the study of american history?

Eric Foner believed that freedom is an effective theme to focus on in the study of American history because of what happened in the past during the American civil war the nation faced many controversial issues such as slavery. Because one of the problems of this American period was slavery. Furthermore, over three million African-Americans were slaves in the South and this means that they worked for other people but had no freedom and received no money for their work.