The correct answer is "War broke out when white settlers pressured the government to take the land from American Indians. "
An example of this would be the Wounded Knee Massacre. This was a result of the discovery of gold on the Lakota Pine Ridge reservation in South Dakota. US citizens wanted to take this land from American Indians, as gold was a valuable commodity that could be sold to make a huge profit. Along with this, the American economy still relied on the gold standard.
This resulted in an armed conflict between members of the US military and the Lakota tribe.