You need to revisit your American history. Colonizers massacred natives through infectious disease (sometimes deliberately, usually not), direct conflict, slaughter of the buffalo they depended on for food, forced relocation to useless land, and other violent tactics. Any natives who were in the way of westward expansion were neutralized, one way or another.

Colonizers destroyed the way of life of those who lived through a deliberate program of "civilization" (reeducation) which included sending native children to boarding schools to be raised in American culture and "rescued" from their own.

Indian removal [0] is a decent place to start reading.

This was not cultural change via the melting pot, this was genocide and reeducation as state policy, with a generous helping of biological warfare.

[0] https://en.wikipedia.org/wiki/Indian_removal