What were the most significant causes for native american losing their land and way of life?

The U. S Military, land and Oil

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.

Answer:Most scholars agree that diseases introduced from the Eastern Hemisphere, including smallpox, measles, and influenza, were the overwhelming cause of population decline (Cook, 1998). The relationship between epidemic disease and American Indian population decline is relatively well documented in the nineteenth century.