US government pursue a two-pronged policy of physical and cultural genocide

Why did the US government pursue a two-pronged policy of physical and cultural genocide toward Native Americans? Why was assimilation so important in relation to American Indians at a time when other ethnic minorities were segregated? How did Native Americans push back against these policies of genocide/assimilation?
http://www.americanyawp.com/text/17-conquering-the-west/
(Primary sources linked at the end of the chapter to use in the essay. Paper minimum 2 pages, maximum 3 pages. Thank you.)

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