Indians are defeated by the Spanish

Illustration after Joannes Stradanus. From Girolamo Benzoni's Americae...,Insignis & Admiranda Historia de reperta primum Occidentali India... Frankfurt: Johann Theodor De Bry, 1594-1619. Second German edition.

Franciscus de Vitoria (1480-1536),  De Indis et de iure belli relectiones

"Again, although Sylvester (Mazzolini, also Prieras, Theologian, b. at Priero, Piedmont, 1460; died in Rome, 1523) discourses at great length on the power of the Pope, yet, under the word infideles (§ 7), he expressly maintains against Hostiensis that unbelievers can not be compelled by arms to recognize this lordship and can not be deprived of their property on this pretext. And Innocent maintains the same in X 3.34.8 (Quod super his). There is also no doubt that this was the opinion of St. Thomas too (Secunda Secundae, qu. 66, art. 8, on obj. 2)."
Bartolomé de Las Casas (1484-1566), De thesauris in Peru. Las Casas wrote De thesauris in 1565 when he was ninety-one years old.

 Three Arguments:

Quod omnes tangit ab omnibus approbari debet

Due Process

Spanish Ownership Contracts of native Slaves were not valid

Brian Tierney, The Idea of Natural Rights: Studies on Natural Rights, Natural Law and Church Law 1150-1625 (Emory University Studies in Law and Religion. Atlanta, Georgia: Scholars Press, 1997) 280-286.