Gold!

The Nez Perce were living peacefully on the reservation when somebody found gold. Immediately, a flood of settlers moved onto the reservation. The government made some small effort at the beginning to stop people but there were too many people and too little military in the area to keep people out, and so they flooded in. Though public and government opinion agreed that the miners were there illegally, instead of trying to get them to move out, they decided instead that the old treaty was untenable and that they would have to renegotiate and force the Nez Perce onto a smaller reservation. If this doesn't sound like a recipe for war, I don't know what would.