After World War I, American people and the authors among them were left disillusioned by the effects that war had on their society. America needed a literature that would explain what had happened and what was happening to their society. American writers turned to what is now known as modernism. The influence of 19th Century realism and naturalism and their truthful representation of American life and people was evident in post World War I modernism. This paper will try to prove this by presenting the basic ideas and of these literary genres, literary examples of each, and the