Why were the Native Americans and white settlers unable to live peacefully on the great plains?

Most conflict between Europe and those decedents stemmed from a series of events, in no particular order.

1. Broken treaties, promises (dishonorable conduct, one of the highest Indian laws common to all tribes, punishable by death)
2. Constant aggression by European settlers
3. Invasions upon native lands (people breaking Indian law on Indian lands)
4. constant encroachment and pushing back the native populations
5. unfair treatment and trade negotiations
6. Failure of the natives to recognize the Catholic Church, or Rulers in Europe as divine, or supreme rulers (from De Indius De Jure Bella (of Indians and Lawful war) F. Victoria, 1532)

This list is not exhaustive, but gives some of the main points of conflict.

But here are some more reasons that caused this conflict:

1. The measles outbreak was blamed on Marcus Whitman.
2. Walla Walla Chief's son died.
3. Protestant and Presbytarian was being forced upon the Cayuse.