Dum Diversas was issued by Nicholas V on 18 June 1452 and addressed to Afonso V of Portugal. Its operative clause gives the king full and free faculty to invade, search out, capture, vanquish and subdue all Saracens and pagans and other enemies of Christ wherever they may be, to reduce their persons to perpetual servitude, and to take their kingdoms, possessions and goods and convert them to his own use.
The immediate setting was a crusading one. Constantinople was under pressure and would fall the following year, and the papacy was trying to hold a coalition together against Ottoman expansion. Portugal's Atlantic voyages down the West African coast were presented, and paid for, as an extension of that effort. The bull draws no line between war against a Muslim power in the Mediterranean and the seizure of people on a coast where no war existed.
Read at the time as a war measure, the language became something else in practice. Portuguese ships had been taking captives from the Senegambian coast since the 1440s, and the traffic grew steadily after 1452. The bull supplied a formula that could be repeated: unbelief made a people liable to conquest, and conquest carried title to their persons and their property. Three years later Romanus Pontifex confirmed and widened it, and in 1493 Inter Caetera applied the same reasoning across the Atlantic. Together the three texts are what later lawyers and campaigners have summarised as the doctrine of discovery.