B. War broke out when white settlers pressured the government to take the land from American Indians is your answer.
The discovery of gold in the Native American Territory led to a huge gold rush, in which large amounts of US citizens moved westwards to try to strike rich. However, most of the gold were found in territory that was set aside for the Native's usage, and when the gold-seekers started streaming in, it was seen as a further take-over of their ancestoral lands by the US government. Constant pleading fell into deaf ears, and so the Native's acted on their own behalf and (in a sense) declared war against the settlers. This led to the constant on-and-off war between the US and most Native American tribes and resulted in the tribes being moved to reservations.
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