What led to the decline of the native americans?

1. disease brought by colonists was a major factor. Kills millions
2. violent conflicts between the colonists and/or British forces against the natives
3. The westward expansion (there's a special name for it) Colonists kept colonizing deeper and deeper into the mainland until reaching the western sea coast. killing 10's of thousands on the way there
*millions died, possible around 10 mill