By the summer of 1452 Ottoman Sultan Mehmed II had completed the Rumelihisarı fortress on the western or European side of the Bosphorus. Located several miles north of Constantinople, it commanded the narrowest part of the Bosporus. Byzantine Emperor Constantine XI wrote to Pope Nicholas for help. Issued less than a year before the Fall of Constantinople in 1453, the bull may have been intended to begin another crusade against the Ottoman Empire. Nicholas V's nephew, Loukas Notaras, was Megas Doux of the Byzantine Empire. It was not until Alfonso V of Portugal responded to a Papal call for aid against the Turks, that Pope Nicholas V agreed to support the Portuguese claims regarding territory in Africa. Although some troops did arrive from the mercantile city states in the north of Italy, Pope Nicholas did not have the influence the Byzantines thought he had over the Western Kings and Princes. France and England were both weakened by the Hundred Years' War, and Spain was still engaged in conflict with Islamic strongholds in Iberia. Any Western contribution was not adequate to counterbalance Ottoman strength.
In mid-fifteenth-century Portugal, the ideals of chivalric honor and crusading were seen as the path for ambition and success. During the reign of Afonso V, the Portuguese nobility enjoyed great influence and prestige, and for several decades the house of Bragança was the wealthiest and most influential force in the kingdom. In 1415 the wisdom and justice of an attack on Morocco had to be seriously weighed, but during the reign of Afonso V and for the century following, "such enterprises were accepted as self-justifying crusades for religion, chivalry, and honor".
The raids and attacks of the Reconquista created captives on both sides, who were either ransomed or sold as slaves. The Portuguese crown extended this to North Africa. After the attack on Ceuta, the king sought papal recognition of it as a crusade. Similarly, after the 1441 attack on Mauretania, the crown again sought after the fact, papal acknowledgement that this was part of a just conflict. Such a determination would then indicate that those captured could legitimately be sold as slaves.