Portugal's maritime expansion began in 1415 when João I seized Ceuta in Morocco, the western depot for the spice trade. The military campaign against Ceuta was launched for several reasons. First, war in Morocco was seen as a new crusade against the Muslims that would stand Portugal well with the church. Second, there was a need to suppress Moroccan pirates who were threatening Portuguese ships. Third, the Portuguese wanted the economic benefit that controlling Ceuta's vast market would bring to the crown. Finally, the campaign against Ceuta was seen as preparatory to an attack on Muslims still holding Granada. The possession of Ceuta allowed the Portuguese to dominate the Straits of Gibraltar.
After the conquest of Ceuta, Prince Henry the Navigator, who had participated in the campaign as an armed knight, settled at Sagres on the extreme end of Cape St. Vincent, where in 1418 he founded a naval school. He continued to direct Portugal's early maritime activity. As the master of the Order of Christ, Prince Henry was able to draw on the vast resources of this group to equip ships and pay the expenses of the early maritime expeditions. Prince Henry was motivated by scientific curiosity and religious fervor, seeing the voyages as a continuation of the crusades against the Muslims and the conversion of new peoples to Christianity, as well as by the desire to open a sea route to India.
Shortly after establishing his school, two of Prince Henry's captains discovered the island of Porto Santo, and the following year the Madeira Islands were discovered. In 1427 Diogo de Silves, sailing west, discovered the Azores archipelago, also uninhabited. Both Madeira and Porto Santo were colonized immediately and divided into captaincies. These were distributed to Prince Henry's captains, who in turn had the power to distribute land to settlers according to the Law of the Sesmarias.
Prince Henry's plan required the circumnavigation of Africa. His early voyages stayed close to the African coast. After repeated attempts, Gil Eanes finally rounded Cape Bojador on the west coast of Africa in present-day Western Sahara in 1434, a psychological, as well as physical, barrier that was thought to be the outer boundary of the knowable world. After passing Cape Bojador, the exploration of the coast southward proceeded very rapidly. In 1436 Gil Eanes and Afonso Baldaia arrived at the Senegal River, which they called the River of Gold when two Africans they had captured were ransomed with gold dust. In 1443 Nuno Tristão arrived at the Bay of Arguin off the coast of present-day Mauritania. These voyages returned African slaves to Portugal, which sparked an interest in the commercial value of the explorations, and a factory was established at Arguin as an entrepôt for human cargo. In 1444 Dinis Dias discovered the Cape Verde Islands, then heavily forested, and Nuno Tristão explored the mouth of the Senegal River. In 1445 Cape Verde was rounded, and in 1456 Portuguese arrived at the coast of present-day Guinea. The following year, they reached present-day Sierra Leone. Thus, when Prince Henry died in 1460, the Portuguese had explored the coast of Africa down to Sierra Leone and discovered the archipelagoes of Madeira, the Azores, and the Cape Verde Islands.
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