These people slaughtered millions of buffalo taking away the livelihood of
the native tribes.

Buffalo soldiers explanation:
But then white trappers and traders introduced guns in the West, killing millions more buffalo for their hides. By the middle of the 19th century, even train passengers were shooting bison for sport. "Buffalo" Bill Cody, who was hired to kill bison, slaughtered more than 4,000 bison in two years.