Answer: War broke out when white settlers pressured the government to take the land from American Indians.
Due to the California gold rush (1848-1855), about 300.000 people migrated to San Francisco to look out for gold. The problem for them was that many places were already inhabited by native americans, which refused to give out their land to white people just so they can take all the gold from it. Of course, settlers pressured the government to fight against the warring tribes in order to have access to more and more gold, which resulted in a lot of conflicts between Native Americans and white people, like the Great SIoux War of 1876.