Jehovah’s promise that he will fulfill his covenants comes crashing down on the Nephites. Jehovah declares that “I will afflict thy seed by the hand of the gentiles.” There is a covenant given to the Nephites, and it promises prosperity and safety if they are righteous. Prophecy, and perhaps current conditions, declare that they will not always be righteous. Thus, they will be afflicted by the gentiles.
A problem with understanding these verses is the use of the word gentile. Jacob has previously used that term to describe the indigenous populations who had joined with the Nephites. While that was a convenient way to discuss internal issues, there remain innumerable indigenous peoples who have not joined with the Nephites. In its description of all who are outsiders, the term gentile refers to anyone who is not Nephite—in the same way that Jacob will declare the use of the term Lamanite in Jacob 1:14.
Jehovah also declares that in addition to afflicting their seed, these future gentiles will have their hearts softened to support the Nephite seed. This again reprises the Isaiah passage about the gentiles being the nursing fathers and nursing mothers to Israel.
The beginning wherefore in verse 19 links the description of what will happen with the way in which Jehovah fulfills his covenant. The promise of the land will continue, and it will become inclusive of all who join into the covenant associated with that promise.