In late spring of 1452 Byzantine Emperor Constantine XI wrote to Pope Nicholas for help against the impending siege by Ottoman Sultan Mehmed II. Nicholas issued the bull Dum Diversas (18 June 1452) authorizing King Afonso V of Portugal to “attack, conquer, and subjugate Saracens, pagans and other enemies of Christ wherever they may be found”. Issued less than a year before the fall of Constantinople, the bull may have been intended to begin another crusade against the Ottoman Empire.

The bull conferred exclusive trading rights to the Portuguese between Morocco and the Indies with the rights to conquer and convert the inhabitants. A significant concession given by Nicholas in a brief issued to King Alfonso in 1454 extended the rights granted to existing territories to all those that might be taken in the future. Consistent with these broad aims, it allowed the Portuguese “to invade, search out, capture, vanquish, and subdue all Saracens and pagans whatsoever, and other enemies of Christ wheresoever placed, and the kingdoms, dukedoms, principalities, dominions, possessions, and all movable and immovable goods whatsoever held and possessed by them and to reduce their persons to perpetual slavery”. However, together with a second reference to some who have already been enslaved, this has been used to suggest that Nicholas sanctioned the purchase of black slaves from “the infidel”: “… many Guineamen and other negroes, taken by force, and some by barter of unprohibited articles, or by other lawful contract of purchase, have been … converted to the Catholic faith, and it is hoped, by the help of divine mercy, that if such progress be continued with them, either those peoples will be converted to the faith or at least the souls of many of them will be gained for Christ.”