The Spanish theologian and political theorist Francisco de Vitoria (ca. 1483-1546) was the first great theorist of modern international law. He provided an updated, if uneasy, justification for Spain's conquests in the New World.

Little is known of the early life of Francisco de Vitoria. He studied at Burgos and taught at the universities of Valladolid (1523-1526) and of Salamanca. At the latter institution, in 1539, he delivered his famous lectures on law, war, and the New World, eventually published as De Indis et de jure belli relectiones (On the Indians and the Law of War).

As a Dominican friar, Vitoria was deeply involved with the teachings on theology and politics of his great predecessor St. Thomas Aquinas. Yet there were worlds of difference between the Mediterranean-centered civilization of the 13th-century Angelic Doctor and the ocean-spanning Hapsburg Empire of Vitoria's day. Vitoria and his colleagues at Salamanca undertook to...