"To make the time frame abundantly clear to both the Nephites and to us, the Lord explains in passing that the Gentiles will come to "this land." The full description of them coming is set out in the earlier prophecy of Nephi as set out at length in First Nephi. (1 Nephi 13 & 14.) But here Christ reminds the audience that when the Gentiles come, they will "scatter my people who are of the house of Israel."
"Gentiles certainly did come. They did scatter the remnants who were on the American continent. Not only did they scatter them, but they also "cast out" and "trodden down" those populations who were here when the Gentiles arrived. Smallpox wiped out the Great Plains Indians. There were an estimated 20 million plus Plains Indians when Columbus arrived. Smallpox all but annihilated them. So few survived that by the time of the western push of the United States, it was believed the Great Plains had never been populated.
"To say they were "trodden underfoot" is descriptive. The native populations were destroyed. They were conquered. They died. Their remains returned to the earth upon which the Gentiles trod.
"You must keep this image in mind as you read about the future of the Gentiles being trodden underfoot. We will get to that later in this same prophecy by Christ." (Denver Snuffer, 3 Nephi 16: 8)