Answer:
The U.S Military, land and Oil
Step-by-step explanation:
The U.S Military needed land throughout America, so when the Native Americans occupied the land that they required to back then construct forts (which still hold up to this day for the most part), the military were forced to knock down their camps and livelihoods, this lead to retaliation and backlash from the natives of the area, leading to many lives lost. If the military didn't do it for land, they did it because the area had oil deposits that they needed to extract.