The United States, like all nations, was created through territorial conquest. Most of its current territory was occupied or frequented by human beings before the U.S. came; the U.S. used force to either displace, subjugate, or kill all of those people. To the extent that land “ownership” existed under the previous inhabitants, the land of the U.S. is stolen land.
This was also true before the U.S. arrived. The forcible theft of the land upon which the U.S. now exists was not the first such theft; the people who lived there before conquered, displaced, or killed someone else in order to take the land. The land has been stolen and re-stolen again and again. If you somehow destroyed the United States, expelled its current inhabitants, and gave ownership of the land to the last recorded tribe that had occupied it before, you would not be returning it to its original occupants; you would simply be handing it to the next-most-recent conquerors.
If you go back far enough in time, of course, at some point this is no longer true. Humanity didn’t always exist; therefore for every piece of land, there was a first human to lay eyes on it, and a first human to say “This land is mine.” But by what right did this first human claim exclusive ownership of this land? Why does being the first person to see a natural object make you the rightful owner of that object? And why does being the first human to set foot on a piece of land give your blood descendants the right to dispose of that land as they see fit in perpetuity, and to exclude any and all others from that land? What about all the peoples of the world who were never lucky enough to be the first to lay eyes on any plot of dirt? Are they simply to be dispossessed forever?