What factors led to the transformation of the trans-Mississippi west after the civil war?

First, it all began with the conflict between the settlers and the indian tribes. There were a lot of of massacres that being done to the native tribes, resulting in thousands of death (such as Sand Creek Massacre or little big horn battle).

Since the settlers generally won the battle, they created Dawes Severalty Act, which dismantled the tribes and make them assimilate among the settlers.

This provide the opportunity for the railroad company to come in and transform the trans - Mississippi west