If God’s work and glory is to bring to pass the immortality and eternal life of man (Moses 1:39), it is in this context that we must examine his reasons for separating the Lamanites from the Nephites. To any serious student of the Old Testament this doctrine will not be surprising. The Old Testament is filled with instances in which the advice of the prophets or the commandments from the Lord are to not intermarry with the heathen nations around them. The reason for this is not to engender racism but to keep these interfaith marriages from destroying the religion of the children of Israel.
Therefore, as the Israelites were settling in the land of Canaan, they were commanded to drive out the nations which were already in the land. This is because of the great wickedness of these nations (see commentary for 2 Nephi 5:21). Had the Israelites made a regular practice of marrying with these peoples it would have proved their destruction. The typical pattern is that the heathen and idol worship practices of the one partner overcome the faith and good intentions of the worshiper of the Lord. This is why the Lord commanded them to drive them out of Canaan, with the warning:
if ye do in any wise go back, and cleave unto the remnant of these nations, even these that remain among you, and shall make marriages with them, and go in unto them, and they to you;
Know for a certainty that the LORD your God will no more drive out any of these nations from before you; but they shall be snares and traps unto you, and scourges in your sides, and thorns in your eyes, until ye perish from off this good land which the LORD your God hath given you. (Josh 23:12-13)
The Lord understands the great influence a wicked and idolatrous spouse can have on a righteous individual. With all the wisdom Solomon had, he was not wise enough to stay faithful to the Lord when his many idolatrous wives began to influence him, when Solomon was old…his wives turned away his heart after other gods: and his heart was not perfect with the Lord his God (1 Kings 11:4). It would have been just the same if the Nephites had not been physically separated from the Lamanites. The Lamanites would have turned away their hearts after their incorrect traditions.