Final answer:
The advancement of military technology and improved reporting through newspapers and cinema newsreels were key in helping Americans understand the horrors of war, particularly during the Civil War and World War I.
Step-by-step explanation:
The invention that greatly contributed to Americans understanding the horrors of war was the advancement of military technology combined with improved communication platforms such as newspapers and cinema newsreels. During the Civil War, technologies like the electric telegraph and logistics innovations using railroads and steamboats allowed for faster and more widespread reporting of the conflict's brutal realities.
By World War I, further advancements in military technology, including machine guns, aircraft, and chemical warfare, contributed to a scale of death and destruction that was previously unimaginable. The surviving soldiers brought home physical and psychological scars, and their stories, compounded by media reports, began to paint a sobering picture of warfare. The media played an increasingly important role by bringing graphic details from the frontlines to the public via newsreels shown in movie theaters and extensive newspaper coverage.
Collectively, these elements transformed public perception of warfare from an idealized narrative of heroism to one more reflective of the grim and tragic realities of modern conflict.