in the bull is not explicit, but historian Richard Raiswell finds that it clearly refers to the recently discovered lands along the coast of West Africa. Portuguese ventures were intended to compete with the Muslim trans-Sahara caravans, which played a key role in the highly profitable Muslim slave trade and also held a monopoly on West African gold and ivory. The Portuguese claimed territorial rights along the African coast by virtue of having invested time and treasure in discovering it; the Castilian claim was based on their being the heirs of the Visigoths. In 1454 a fleet of caravels from Seville and Cádiz traded along the African coast and upon their return, were intercepted by a Portuguese squadron. 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 papal bull Romanus Pontifex, issued on 8 January 1455, endorsed Portuguese possession of Cuerta (which they already held), and the exclusive right to trade, navigation, and fishing in the discovered lands, and reaffirmed the previous Dum Diversas. It granted permission to Afonso and his heirs to "... make purchases and sales of any things and goods, and victuals whatsoever, as it may seem fit, with any Saracens and infidels in said regions; ... provided they be not iron instruments, wood used for construction, cordage, ships, and any kinds of armor." 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,