The Just War theorist Francisci de Victoria (1486?-1546) makes a strong case for going to war to liberate the oppressed from other nations [not an endorsement of his Roman Catholicism, or all the actions of the Conquistadors]:
Another possible title is founded either on the tyranny of those who bear rule among the aborigines of America or on the tyrannical laws which work wrong to innocent folk there, such as that which allows the sacrifice of innocent people or the killing in other ways of uncondemned people for cannibalistic purposes. I assert also that without the Pope's authority the Spaniards can stop all such nefarious usage and ritual among the aborigines, being entitled to rescue innocent people from an unjust death. This is proved by the fact that “God has laid a charge on every individual concerning his neighbor,” and they all are our neighbors. [Editor’s note: In the previous sentence
cites from the non-inspired Apocrypha, however, this principle is still found in Scripture.] Therefore, any one may defend them from such tyrannical and oppressive acts, and it is especially the business of princes to do so. Vitoria
A further proof is given by Proverbs, ch. 24: “Deliver them that are drawn unto death, and forbear not to free those that are being dragged to destruction.” This passage is not to be taken as applying only when victims are actually being dragged to death, but the natives can also be compelled to abstain from such ritual. And if they refuse, it is a good ground for making war on them and proceeding against them under the law of war, and if such sacrilegious rites can not otherwise be stopped, for changing their rulers and creating a new sovereignty over them. In this connection we find the opinion of Innocent and the Archbishop to be sound, namely, that punishment can be inflicted for sins against nature. And it is immaterial that all the Indians assent to rules and sacrifices of this kind and do not wish the Spaniards to champion them, for herein they are not of such legal independence as to be able to consign themselves or their children to death.
 Francisci de Victoria, “On the Lawful Titles Whereby the Aborigines of America could have come into the Power of Spain,” in Francisci de Victoria De Indis et De ivre belli relectiones, trans. John Pawley Bate and ed. Ernest Nys (Carnegie Institution of Washington, 1917), 159.