At this point in Nephite history, strife between Nephites, Lamanites, and Amlicites became so severe that war broke out. Mormon took this occasion to explain how the warriors were able to distinguish themselves from one another in battle. Similar to the red coats worn by the British army and the blue coats worn by the French to distinguish them in battle, the Lamanites and their allies, the Amlicites, marked themselves: "[T]he Amlicites were distinguished from the Nephites, for they had marked themselves with red in their foreheads after the manner of the Lamanites." In addition, the Lamanites shaved the hair on their heads, but the Amlicites chose not to shave their heads. Verse 5 explains that the Lamanites were naked except for a skin which girded their loins. The next verse states, "[A]nd the skins of the Lamanites were dark."
Mormon explained that a mark was set upon the Lamanites "that their seed might be distinguished from the seed of their brethren, that thereby the Lord God might preserve his people, that they might not mix and believe in incorrect traditions which would prove their destruction." The Nephites were generally discouraged from intermarriage with the Lamanites. However, as verse 11 states, "[W]hosover would not believe in the tradition of the Lamanites, but believed … in the commandments of God and kept them, were called the Nephites, or the people of Nephi, from that time forth."
Alma 3 is often cited as evidence of racism in the Book of Mormon. However, when reading ancient historical texts, such as the Book of Mormon, it is absolutely essential not to impose modern ideas of race and cultural identity onto the people of the past. There are several explanations for the mark or curse of the Lamanites—other than racism—when reading this chapter in its entirety.
Remember that the setting for these verses was at a time when the Nephites, Lamanites, and Amlicites were involved in bloody battles against one another. At time of war, it may have been unthinkable to marry someone who is or has viciously fought against you and your people. In fact, this may have been viewed as an act of treason.
Additionally, Mormon appears to couch the issue of the Lamanite curse in terms of religious and cultural identity, not merely skin pigmentation. He records that any person who was "led away by the Lamanites" had the same "mark set upon him." On the other hand, "whosoever would not believe in the tradition of the Lamanites …were called the Nephites, or the people of Nephi." Therefore, the curse of the Lamanites included the ultimate outcome that they would believe "in incorrect traditions which would prove their destruction." There was concern that introducing incorrect traditions and beliefs in the Nephite community by intermarriage could result in the destruction of the Nephite nation as well. These things were done to preserve a nation of people who believed in God and his commandments and had nothing to do with what we would call racism.
When chapter 3 is read in its entirety, it becomes apparent that there may be another explanation for the dark "skins" in question. The dark "skins" were possibly animal skins worn as symbolic clothing, not their normal flesh. This is seen in Mormon’s apparent description of the "skins" being garments worn by the Lamanites. In this sense, the Lamanites and Amlicites were distinguishing themselves by the things they chose to wear or put upon themselves. Thinking they were marking themselves courageously, they unwittingly marked themselves in a way that signaled that they had come out in rebellion, not against their political opponents but against God. Verse 11 explains, "Now the Amlicites knew not that they were fulfilling the words of God when they began to mark themselves in their foreheads [with red markings]; nevertheless they had come out in open rebellion against God; therefore it was expedient that the curse [of separation from God] should fall upon them."
Book of Mormon Central, "Why Did Book of Mormon Prophets Discourage Nephite-Lamanite Intermarriage?" KnoWhy 110 (May 30, 2016).