Answer:
When the Americans expanded the United States, they expanded at the expense of the indigenous people (or Native Americans). As the Americans were expanding, they were forcing those who lived here to leave.
When those who refused to leave were being stubborn, they would be killed, beaten, enslaved, etc. The Native American's culture was completely ruined because of the expansion.
Later on, they did make an agreement to keep some reservations for the natives, where they can still be free and have their own culture and laws.
I hope this helps :)