It was not until Alfonso V of Portugal responded to a Papal call for aid against the Turks, who eventually seized Constantinople in 1453, that Pope Nicholas V supported the Portuguese claims in the bull Dum Diversas.
Nonetheless, in 1454 a fleet of caravels from Seville and Cadiz traded along the African coast and upon their return, were intercepted by a Portuguese squadron. One of the ships was captured with crew and cargo taken to Portugal. Enrique IV of Castile threatened war. Afonso V appealed to the Pope for moral support of Portugal's right to a monopoly of trade in lands she discovered.
The bull, issued in January 1454, endorsed Portuguese possession of Ceuta (which they already held), and the exclusive right to trade, navigation, and fishing in the discovered lands. It provided an exemption from a Canon Law prohibition of trading with infidels.