If you would like to make your answer brief, you might consider just mentioning Sepulveda's views on the intellectual and moral inferiority of the Native peoples as the main reason the Spanish considered the conquest of such populations necessary during the Columbian exchange.
According to Juan de Sepulveda in Democrates Alter, Or, On The Just Causes For War Against the Indians, there are four reasons for a just war:
1) When all other avenues for peace have been exhausted, utilizing necessary force to repel hostile force is justified for the purposes of preserving one's civilization.
2) To recover property seized unjustly.
3) To punish evil-doers who have not been properly dealt with in order to prevent re-occurrence of crimes.
4) To subjugate a race who Sepulveda says 'by natural condition must obey others and refuse to do so.'
This is basically the exchange of people, livestock, plants, and even diseases between the Old World (Europe) and the New World (The Americas). Foods brought to the New World included cotton, wheat, rye, barley, cotton, tobacco, sugar, bananas, and varieties of citrus fruits. Animals brought to the New World included horses, cattle, pigs, goats, and sheep. Unfortunately, the Europeans also brought diseases such as measles, mumps, small-pox, influenza, and chicken pox to the Americas. The Native Americans had no immunity against such diseases. Epidemic after epidemic wiped out sizable Native American populations in the New World.
This led to the importation of African slaves to the New World to bolster the labor component on many emerging farms and plantations. So, how was this importation of foreign slaves justified, and how do Sepulveda's views fit into all this? You might recall my earlier mention that one of Sepulveda's views on the just war was the right to subjugate people termed intellectually and morally inferior. Here's an example from the text which supports what I am saying. It explains why Sepulveda thinks it was perfectly acceptable to subjugate both African and Native American populations.
Compare, then, these gifts of prudence, talent, magnanimity, temperance, humanity, and religion with those possessed by these half-men (homunculi), in whom you will barely find the vestiges of humanity, who not only do not possess any learning at all, but are not even literate or in possession of any monument to their history except for some obscure and vague reminiscences of several things put down in various paintings; nor do they have written laws, but barbarian institutions and customs. Well, then, if we are dealing with virtue, what temperance or mercy can you expect from men who are committed to all types of intemperance and base frivolity, and eat human flesh?
Sepulveda cites the example of the inhabitants of Mexico (then a colony of Spain that he calls New Spain) who live as 'employees of the king, paying, thanks to him, exceedingly high taxes...' He reasons that they should have no problems exchanging such 'barbarous and impious and inhuman' rulers for their Spanish overlords who are 'Christians, cultivators of human virtues and the true faith..' Sepulveda felt that resistance against what he termed the barbarity and cannibalism of Native Americans was justified and
that the Spanish have a perfect right to rule these barbarians of the New World and the adjacent islands, who in prudence, skill, virtues, and humanity are as inferior to the Spanish as children to adults, or women to men, for there exists between the two as great a difference as between savage and cruel races and the most merciful, between the most intemperate and the moderate and temperate and, I might even say, between apes and men.
Thanks for the question. Hope this helps!