Final answer:
The best summary of a theme of 'A Horseman in the Sky' is the impact of war on individuals, as the story illustrates a soldier's inner turmoil and personal conflict during war.
Step-by-step explanation:
A Horseman in the Sky by Ambrose Bierce is a story that best summarizes the theme of the impact of war on individuals. This theme is explored through the story's depiction of a young soldier's dilemma when he's faced with a daunting decision that challenges his loyalties and moral compass during the American Civil War. It showcases the tragic circumstances and personal conflicts that arise from wartime situations, demonstrating how war can profoundly affect the psyche and decisions of individuals.