In which case, policy, or situation did the motto "kill the indian, but save the man" play an important roll in western expansion

One of the clearest policy manifestations of the "kill the Indian, save the man" concept in western expansion would be those of the boarding school era. These policies removed Native American children from their homes and sent them to far-off boarding schools in an effort to replace (and remove) Native languages, customs, and culture from an entire generation. White policymakers waged a cultural genocide on the generation in an effort to replace their Native traditions with English, Christianity, and other white, Euroamerican values. The earliest boarding schools were actually created by William Pratt, the military official who first coined the "kill the Indian, save the man" motto.

Americans agreed to the settlement terms established by the mexican government.

In the midwest because it has more moderate climate than the north or south west which makes farming far easier.