Final answer:
World War I was pivotal in shaping early 20th-century American literature, introducing new themes, firsthand experiences, and a questioning of traditional values, leading to the Modernist movement and a transformation of cultural norms.
Step-by-step explanation:
World War I was considered the most influential force on American writers of the early 20th century because the war marked a turning point in American cultural values. It introduced new literary themes and styles, writers experienced the war firsthand, and war-related trauma spurred a surge in creative expression. The era surrounding WWI, known as Modernism, involved a significant redefinition of America's political, religious, economic, and social values. This period was marked by social upheaval, extraordinary growth, and accelerated change, reflecting in a literary movement where tradition was questioned, and new ways of thinking took root. Writers such as Ernest Hemingway—who served as an ambulance driver during the war—were directly influenced by their experiences, shaping their perspectives and the narratives they penned. The conflict was unprecedented in its scale and destruction, leading to a generation of disillusioned and contemplative artists who sought to convey their sense of betrayal and the new realities of a transformed world.