Manifest Destiny is one of those heinous beliefs, partially supernaturally derived, that newcomers to the Americas had a right and duty to expand and change the land and peoples of the Americas so as to comply to their "real" beliefs and ways of living. It is a pernicious belief that still pervades American (and honestly, other countries) acts of nationalism into the modern era. In that form or another it is effective propaganda, serving to allow certain people in removing responsibility for their actions. After all, the Universe itself says it's okay, they know it and feel it and are told by their leaders it is so.
The 1872 painting American Progress by by John Gast reveals much of the prevailing attitude of the settlers. While technology, knowledge and advances in transportation and agriculture are good for a civilization so too was it a virtue to erase what came before. Notice the lighting in the painting. As the Spirit of America heads to the west bringing illumination the natives and their lifestyle flee the light of progress, staying to the darkness, unable or unwilling to embrace the new era.