The origin of the Kingdom of Portugal lay in the reconquista, the gradual reconquest of the Iberian peninsula from the Moors.

After establishing itself as a separate kingdom in 1139, Portugal completed its reconquest of Moorish territory by reaching Algarve in 1249, but its independence continued to be threatened by neighbouring Castile until the signing of the Treaty of Ayllón in 1411.

Free from threats to its existence and unchallenged by the wars fought by other European states, Portuguese attention turned overseas and towards a military expedition to the Muslim lands of North Africa. There were several probable motives for their first attack, on the Marinid Sultanate (in present-day Morocco). It offered the opportunity to continue the Christian crusade against Islam; to the military class, it promised glory on the battlefield and the spoils of war; and finally, it was also a chance to expand Portuguese trade and to address Portugal’s economic decline.

In 1415 an attack was made on Ceuta, a strategically located North African Muslim enclave along the Mediterranean Sea, and one of the terminal ports of the trans-Saharan gold and slave trades. The conquest was a military success, and marked one of the first steps in Portuguese expansion beyond the Iberian Peninsula, but it proved costly to defend against the Muslim forces that soon besieged it. The Portuguese were unable to use it as a base for further expansion into the hinterland, and the trans-Saharan caravans merely shifted their routes to bypass Ceuta and/or used alternative Muslim ports.

Although Ceuta proved to be a disappointment for the Portuguese, the decision was taken to hold it while exploring along the Atlantic African coast. A key supporter of this policy was Infante Dom Henry the Navigator, who had been involved in the capture of Ceuta, and who took the lead role in promoting and financing Portuguese maritime exploration until his death in 1460. At the time, Europeans did not know what lay beyond Cape Bojador on the African coast. Henry wished to know how far the Muslim territories in Africa extended, and whether it was possible to reach Asia by sea, both to reach the source of the lucrative spice trade and perhaps to join forces with the fabled Christian kingdom of Prester John that was rumoured to exist somewhere in the “Indies”. Under his sponsorship, soon the Atlantic islands of Madeira (1420) and Azores (1427) were reached and started to be settled producing wheat to export to Portugal.

Fears of what lay beyond Cape Bojador, and whether it was possible to return once it was passed, were assuaged in 1434 when it was rounded by one of Infante Henry’s captains, Gil Eanes. Once this psychological barrier had been crossed, it became easier to probe further along the coast.^[14] In 1443 Infante Dom Pedro, Henry’s brother, granted him the monopoly of navigation, war and trade in the lands south of Cape Bojador. Later this monopoly would be enforced by the Papal bulls Dum Diversas (1452) and Romanus Pontifex (1455), granting Portugal the trade monopoly for the newly discovered lands. A major advance which accelerated this project was the introduction of the caravel in the mid-15th century, a ship that could be sailed closer to the wind than any other in operation in Europe at the time. Using this new maritime technology, Portuguese navigators reached ever more southerly latitudes, advancing at an average rate of one degree a year. Senegal and Cape Verde Peninsula were reached in 1445.

The first feitoria trade post overseas was established in 1445 on the island of Arguin off the coast of Mauritania, to attract Muslim traders and monopolize the business in the routes travelled in North Africa. In 1446, Álvaro Fernandespushed on almost as far as present-day Sierra Leone and the Gulf of Guinea was reached in the 1460s.

Expansion of sugarcane in Madeira started in 1455, using advisers from Sicily and (largely) Genoese capital to produce the “sweet salt” rare in Europe. Already cultivated in Algarve, the accessibility of Madeira attracted Genoese and Flemish traders keen to bypass Venetian monopolies. Slaves were used, and the proportion of imported slaves in Madeira reached 10% of the total population by the 16th century. “By 1480 Antwerp had some seventy ships engaged in the Madeira sugar trade, with the refining and distribution concentrated in Antwerp. By the 1490s Madeira had overtaken Cyprus as a producer of sugar.” The success of sugar merchants such as Bartolomeo Marchionniwould propel the investment in future travels.

In 1469, after prince Henry’s death and as a result of meagre returns of the African explorations, King Afonso V granted the monopoly of trade in part of the Gulf of Guinea to merchant Fernão Gomes. Gomes, who had to explore 100 miles (160 km) of the coast each year for five years, discovered the islands of the Gulf of Guinea, including São Tomé and Príncipe and found a thriving alluvial gold trade among the natives and visiting Arab and Berber traders at the port then namedMina (the mine), where he established a trading post. Trade between Elmina and Portugal grew throughout a decade. In 1481, the recently crowned João IIdecided to build São Jorge da Mina in order to ensure the protection of this trade, which was held again as a royal monopoly. The Equator was crossed by navigators sponsored by Fernão Gomes in 1473 and the Congo River by Diogo Cão in 1482. It was during this expedition that the Portuguese first encountered theKingdom of Kongo, with which it soon developed a rapport. During his 1485–86 expedition, Cão continued to Cape Cross, in present-day Namibia, near theTropic of Capricorn.

In 1488, Bartolomeu Dias rounded the Cape of Good Hope on the southern tip of Africa, proving false the view that had existed since Ptolemy that the Indian Oceanwas land-locked. Simultaneously Pêro da Covilhã, traveling secretly overland, had reached Ethiopia, suggesting that a sea route to the Indies would soon be forthcoming.

As the Portuguese explored the coastlines of Africa, they left behind a series of padrões, stone crosses engraved with the Portuguese coat of arms marking their claims, and built forts and trading posts. From these bases, they engaged profitably in the slave and gold trades. Portugal enjoyed a virtual monopoly on the African seaborne slave trade for over a century, importing around 800 slaves annually. Most were brought to the Portuguese capital Lisbon, where it is estimated black Africans came to constitute 10 per cent of the population.