Assimilation is a process where a certain culture comes to resemble another group and the new members of the society become indistinguishable from members of another group.
The Native American children in the film are forcefully taken to boarding schools which were far away from where they are taught the western domestic chores and their homes and families until they got into adulthood and this made them get assimilated to another culture and they ended up forgetting their native language, and some of them were abused, raped and beaten when they spoke their native language or crossing he school border to force them change from their original culture.
The government didn’t like the Indian culture and therefore it decided to assimilate them into the western culture. In the long term the assimilation affected the Native Americans and because of tough and traumatic life experiences they went through from their childhood until when they were adults, it led to most of them lacking the proper and the acceptable norms of an individual in a society and ended up as prostitutes, drug addicts and alcoholics while others ended up into suicidal acts because they could not fit n the western culture when they grew up.
I don’t think that assimilating others into another culture is ever justified . If the individuals are not willing to be change from their cultural practices and norms they should not be forced to change . Landfills should have the right to interact with anybody from whichever cultural background and learn different cultures but they should be given the to practice their culture and religion.