When we learned in school about the expansion of railroads and white "settlers" to the west, they did mention the slaughter of the bison. They said how a trainload of white men with guns would roll right up to a herd and shoot them all and then just leave the carcasses. It was horrifying, but even so, it wasn't the truth. You see. they portrayed this slaughter-to-the-point-of-near-extinction as mere foolishness or ignorance; that the "hunters" were so excited about the new "prey" and so "caught up in the sport" that they "unthinkingly" were very wasteful and didn't use the meat (because they were "shooting for sport," you see, and not for food) and that it was all just a big "oopsie" that the Plains became empty of bison.
Only MUCH, much later did I learn the awful truth. It was a calculated, on purpose plot by the U.S. Government to wipe out the most major component of the food, clothing, tools, fuel, shelter, culture, etc. of the Plains Indians. With the bison nearly all gone, it was much easier for the Army to move in and send people away, onto reservations, across the country...
Wiped them out ON PURPOSE. Just like purposefully infecting people with smallpox, a virus their immune systems had never seen before, conveniently opened up lots of prime real estate for the invaders.
And it's this mindset of the conqueror, I think, that is the root of white nationalism and racism. I think it's naive to think that racism or white supremacy is about thinking that Black people or Native people or any immigrant people are somehow "inferior." On the contrary, whites know full well that the other groups are not inferior in any way. BUT the whites live in a nation where they have the POWER because they were the invaders and conquerors and enslavers, and they will NEVER give up that power, even though they KNOW they have it illegitimately.
And flying racist flags is one way they signal to "the others:" WE are in power, WE conquered and murdered and displaced and enslaved you and don't you forget it! We WILL protect this power structure and won't hesitate to terrorize or kill to keep it in place, just the way we like it.