The correct answer is - The government had not followed through with promises of help.
The conflict resulted because of treaty violations by the United States government, and also by late or unfair annuity payments by the Indian agents. Throughout the 1950's this led to starvation among the Dakota (Sioux) people, and they were very frustrated about it.
Officially, thee conflict started when few Dakota hunters killed few white settlers that were also hunting, and afterwards the Dakota decided that they should kick out the white settlers from their territory and it is estimated that they killed around 800 men, women, and children. The US government responded with military action, captured lot of Dakota warriors, and some of them executed by hanging.
