Alma 3:16 - 3:20

Alma 3:16 And again: I will set a mark upon him that fighteth against thee and thy seed.

Mormon reiterates that anyone who fights against the Nephites will have a mark set upon them to distinguish them from the Nephites.
I am thinking that this will happen in our day as well. Why else would Mormon tell us this twice?

Alma 3:17 And again, I say he that departeth from thee shall no more be called thy seed; and I will bless thee, and whomsoever shall be called thy seed, henceforth and forever; and these were the promises of the Lord unto Nephi and to his seed.

In fact, anyone who leaves the Nephites to join other groups will not be called Nephites anymore. However, those who remain loyal will be blessed and anyone who joins them will as well. These were the promises made to Nephi by the Lord himself.

Alma 3:18 Now the Amlicites knew not that they were fulfilling the words of God when they began to mark themselves in their foreheads; nevertheless they had come out in open rebellion against God; therefore it was expedient that the curse should fall upon them.

And to reiterate the point again, the Amlicites who have come out in rebellion against the Nephites and had put the red mark on themselves were unknowingly fulfilling prophecy. The end result is that the curse of the Lamanites was now placed on them.

Alma 3:19 Now I would that ye should see that they brought upon themselves the curse; and even so doth every man that is cursed bring upon himself his own condemnation.

The above description is indicative of men do to themselves. Whatever curse or condemnation falls upon a person is of their own making.
Jump into a briar patch, you will get scratched up. The scratches are the natural result of jumping into the briar patch. Note that no one put the scratches on you. You did it to yourself. People may have tried to warn you of the danger of jumping into a briar patch, but you went ahead and did it anyway. Therefore the natural consequences are yours to deal with. The consequences of sin are the natural result of commiting sin. The Lord has tried to warn us all of the danger of committing sin, but since we do so anyway we will suffer the natural consequences of doing so.

Alma 3:20 Now it came to pass that not many days after the battle which was fought in the land of Zarahemla, by the Lamanites and the Amlicites, that there was another army of the Lamanites came in upon the people of Nephi, in the same place where the first army met the Amlicites.

It seems the danger from the enemies of the Nephites is not over yet. Another Lamanite army is marching into the land of the Nephites and is attacking in the same location that Alma and the Nephites first engaged the armies of Amlici.