Explain why and how the relationship between the United States and Native Americans change during the late 1800s?

The relationship between the US Government and the Native American populations has changed drastically during the 1800s because of the continued expansion of the US government. The Native American's settlement was constantly encroached in several regions of the new nation. 
As the relationship soured, armed conflicts escalates this was then called the Indian war. 

Later on, US government and the Native Americans signed the Indian Appropriations Act in 1871 where it states;  
                                       That hereafter no Indian nation or tribe within the territory of the United States shall be acknowledged or recognized as an independent nation, tribe, or power with whom the United States may contract by treaty: Provided, further, that nothing herein contained shall be construed to invalidate or impair the obligation of any treaty heretofore lawfully made and ratified with any such Indian nation or tribe.