Immediately after Lehi tells his sons that they have received a land of promise that will be protected from other nations that would come unto it, he tells them that it won’t last. There will come a time when they will “dwindle in unbelief, after they have received so great blessings from the hand of the Lord.”
The explanation for the nations that had been kept from this new land is answered when Lehi notes that it is only after his seed has dwindled in unbelief will those nations come unto them. After that, there are terrible bloodsheds.
Nephi had already seen this vision of the future, and wrote about it in 1 Nephi 13:7 when he speaks of the gentiles who would come over the many waters. That process of destruction by the gentiles from across the ocean began in the early 1500s as Spain conquered most of the New World. No doubt some of the “great visitations” were the diseases that depopulated much of the New World. It was a sad story that was repeated when virtually every nation of the gentiles crossed the ocean and came upon the land. It devastated not only the place where the Nephites had lived, but it also devastated the entire hemisphere.