1069

I'm gonna guess that none of the nationalists are willing to touch this.
On the surface of it, I guess, the answer is simply, "White people did it because they could."
In the final analysis, it's not so much about what's "right" (morality's subjective) as what's possible.

There were exponentially fewer people on the planet, back when Europeans first colonized America. The competition for land and natural resources was less fierce than it is today.
Many of our ancestors were given land, for free. The mexican government, for instance, gave my family massive tracts of land back in the 1800s when Texas was still part of Mexico, simply because they wanted Texas colonized and cultivated- civilized- and most Mexicans didn't care to live here (indians were a problem) even if their government paid them to do so, so instead the mexican government offered free land grants to anglo settlers.
Ironic, in light of the situation today, not even 200 years later.
Something similar happened when the Western part of the US was still wild; the US government gave settlers free land grants out west. They could just move out there and build a homestead and start farming or ranching or whatever.

But all the land in the US is owned now; there's no more free land.
There's probably not much un-owned land left in the entire world, actually. And there are not, according to many Americans, enough resources here for us to allow immigrants to come in and work. Which seems preposterous to me; it's obvious the US is richer than Croesus.
But I guess we just don't like to share.
We didn't get where we are today by being generous.

__________________
"1069, why do you relish death?"
~ MrVicchio