"You know how the war started by the killing of some white people near Acton, in Meeker County. Some Dakota seized that moment to declare war to reclaim their homelands from the whites who would not keep their promises. In the early morning hours of August 18, they went to war.
The sentence "We are also told that it is on account of this happening that people who have loved each other very much sometimes quarrel and are no longer friends." best indicates the story's message or theme.
