The history of the Kingdom of Portugal from the Illustrious Generation of the early 15th century to the fall of the House of Aviz in the late 16th century has been named the "Portuguese golden age" (Portuguese: Século de Ouro; "golden century") and the "Portuguese Renaissance". During this period, Portugal was the first European power to begin building a colonial empire as during the Age of Exploration Portuguese sailors and explorers discovered an eastern route to India (that rounded the Cape of Good Hope) as well as several Atlantic archipelagos (like the Azores, Madeira, and Cape Verde) and colonized the African coast and Brazil. They also explored the Indian Ocean and established trading routes throughout most of southern Asia, sending the first direct European maritime trade and diplomatic missions to Ming China and to Japan, at the same time installing trading posts and the most important colony: Portuguese Macau (Only in East Asia). The Portuguese Renaissance produced a plethora of poets, historians, critics, theologians, and moralists. The by Garcia de Resende (printed 1516) is taken to mark the transition from Old Portuguese to the modern Portuguese language.

John I John I of Portugal acceded in 1390 and ruled in peace, pursuing the economic development of his realm. The only significant military action was the siege and conquest of the city of Ceuta in 1415. By this step he aimed to control navigation of the African cut in the broader perspective, this was the first step opening the Arab world to medieval Europe, which in fact led to the Age of Discovery with Portuguese explorers sailing across the whole world.

Contemporaneous writers describe John as a man of wit, very keen on concentrating power on himself, but at the same time with a benevolent and kind personality. His love for knowledge and culture was passed to his sons, often collectively referred to by Portuguese historians as the "illustrious generation" (Ínclita Geração): Edward, the future king, was a poet and a writer; Peter, the Duke of Coimbra, was one of the most learned princes of his time; and Prince Henry the Navigator, the duke of Viseu, invested heavily in science and the development of nautical pursuits. In 1430, John's only surviving daughter, Isabella, married Philip the Good, Duke of Burgundy, and enjoyed an extremely refined court culture in his lands; she was the mother of Charles the Bold.

Edward

Under king Edward, the colony at Ceuta rapidly became a drain on the Portuguese treasury, and it was realised that without the city of Tangier, possession of Ceuta was worthless. In 1437, Duarte's brothers Henry and Ferdinand persuaded him to launch an attack on the Marinid sultanate of Morocco. The expedition was not unanimously supported: Infante Peter, Duke of Coimbra, and the Infante John were both against the initiative; they preferred to avoid conflict with the king of Morocco. Their instincts proved to be justified. The resulting attack on Tangier, led by Henry, was a debacle. Failing to take the city in a series of assaults, the Portuguese siege camp was soon itself surrounded and starved into submission by a Moroccan relief army. In the resulting treaty, Henry promised to deliver Ceuta back to the Marinids in return for allowing the Portuguese army to depart unmolested. Duarte's youngest brother, Ferdinand, was handed over to the Marinids as a hostage for the final handover of the city.

Afonso V

Treaty of Tordesillas

Meanwhile, colonization progressed in the Azores and Madeira, where sugar and wine were now produced; above all, the gold brought home from Guinea stimulated the commercial energy of the Portuguese. It had become clear that, apart from their religious and scientific aspects, these voyages of discovery were highly profitable. Under Afonso V (1443–1481), surnamed the African, the Gulf of Guinea was explored as far as Cape St Catherine (Cabo Santa Catarina),

and three expeditions (1458, 1461 and 1471) were sent to Morocco; in 1471 Arzila and Tangier were captured from the Moors. Under John II (1481–1495) the fortress of São Jorge da Mina, the modern Elmina, was founded for the protection of the Guinea trade. Diogo Cão, discovered the Congo in 1482 and reached Cape Cross in 1486; Bartolomeu Dias doubled the Cape of Good Hope in 1488, thus proving that the Indian Ocean was accessible by sea.

After 1492 the arrival of Christopher Columbus in the West Indies rendered desirable a delimitation of the Spanish and Portuguese spheres of exploration. This was accomplished by the Treaty of Tordesillas (June 7, 1494), which modified the delimitation authorized by Pope Alexander VI in two bulls issued on May 4, 1493. The treaty gave to Portugal all lands that might be discovered east of a straight line drawn from the Arctic Pole to the Antarctic, at a distance of 370 leagues west of Cape Verde. Spain received the lands discovered west of this line. The known means of measuring longitude were so inexact, however, that the line of demarcation could not in practice be determined (see J. de Andrade Corvo in Journal das Ciências Matemáticas, xxxi.147–176, Lisbon, 1881), so the treaty was subject to very diverse interpretations. On its provisions were based both the Portuguese claim to Brazil and the Spanish claim to the Moluccas (see History of East Indies). The treaty was chiefly valuable to the Portuguese as a recognition of the prestige they had acquired. That prestige was enormously enhanced when, in 1497–1499, Vasco da Gama completed the voyage to India.

The tendency to secrecy and falsification of dates casts doubts about the authenticity of many primary sources. Several historians have hypothesized that John II may have known of the existence of Brazil and North America as early as 1480, thus explaining his wish in 1494 at the signing of the Treaty of Tordesillas to push the line of influence further west. Many historians suspect that the real documents would have been placed in the Library of Lisbon. Unfortunately, due to the fire following the earthquake of 1755, nearly all of the library's records were destroyed, but an extra copy available in Goa was transferred to Lisbon Tower of Tombo during the following 100 years. The Corpo Cronológico (Chronological Corpus), a collection of manuscripts on the Portuguese explorations and discoveries in Africa, Asia and Latin America, was inscribed on UNESCO’s Memory of the World Register in 2007 in recognition of its historical value "for acquiring knowledge of the political, diplomatic, military, economic and religious history of numerous countries at the time of the Portuguese Discoveries."

Afonsine Ordinances

While the Crown was thus acquiring new possessions, its authority in Portugal was temporarily overshadowed by the growth of aristocratic privilege. After the death of Edward, further attempts to curb the power of the nobles were made by his brother, D. Pedro, duke of Coimbra, who acted as regent during the minority of Afonso V of Portugal (1439–1448). The head of the aristocratic opposition was the Duke of Braganza, who contrived to secure the sympathy of the king and the dismissal of the regent. The quarrel led to civil war, and on 20 May 1449, D. Pedro was defeated and killed at the Battle of Alfarrobeira. Thenceforward the grants made by John I were renewed and extended on so lavish a scale that the Braganza estates alone comprised about a third of the whole kingdom.

An unwise foreign policy simultaneously injured the royal prestige, for Afonso married his own niece, Joanna, daughter of Henry IV of Castile, and claimed the kingdom of Castile in her name. At the Battle of Toro, in 1476, he fought an indecisive battle that made him realize that his claims to the Castilian throne were not achievable. However, Portugal defeated Castile in the naval war (1475–79) of the same conflict (War of the Castilian Succession), capturing a large Castilian fleet -full of gold- in the Battle of Guinea (1478).

In 1479 Ferdinand and Isabella and Afonso signed the Treaty of Alcáçovas, by which Joanna was relegated to a convent and Portugal won the hegemony in the Atlantic Ocean.

His successor, John II (1481–1495), reverted to the policy of matrimonial alliances with Castile and friendship with England. Finding, as he said, that the liberality of former kings had left the Crown "no estates except the high roads of Portugal," he determined to crush the feudal nobility and seize its territories. The Portuguese Cortes held at Évora (1481) empowered judges nominated by the Crown to administer justice in all feudal domains. The nobles resisted this infringement of their rights, but their leader, Fernando II, Duke of Braganza, was beheaded for high treason in 1483; in 1484 the king stabbed to death his own brother-in-law, Diogo, Duke of Viseu; and eighty other members of the aristocracy were executed afterwards. Thus John "the Perfect," as he was called, assured the supremacy of the Crown. He was succeeded in 1495 by Manuel I, who was named "the Great" or "the Fortunate," because in his reign the sea route to India was discovered and a Portuguese Empire founded.

Imperial expansion (1481–1557)

John II (r. 1481–1495) famously restored the policies of Atlantic exploration, reviving the work of his great-uncle, Henry the Navigator. The Portuguese explorations were his main priority in government, pushing south the known coast of Africa with the purpose of discovering the maritime route to India and breaking into the spice trade. Manuel I (r. 1495–1521) proved a worthy successor to his cousin John II, supporting Portuguese exploration of the Atlantic Ocean and the development of Portuguese commerce.

Under John III (r. 1521–1557), Portuguese possessions were extended in Asia and in the New World through the Portuguese colonization of Brazil. John III's policy of reinforcing Portugal's bases in India (such as Goa) secured Portugal's monopoly over the spice trade of cloves and nutmeg from the Maluku Islands. As a result, John III has been called the "Grocer King". On the eve of his death in 1557, the Portuguese empire spanned almost 1 billion acres (about 4 million square kilometers). During his reign, the Portuguese became the first Europeans to make contact with both China, under the Ming Dynasty, and Japan, during the Muromachi period (see Nanban trade). John III abandoned Muslim territories in North Africa in favor of trade with India and investment in Brazil. In Europe, he improved relations with the Baltic region and the Rhineland, hoping that this would bolster Portuguese trade.

Sebastian I Sebastian of Portugal was the penultimate Portuguese monarch of the House of Aviz. He was the son of John Manuel, Prince of Portugal, and his wife, Joanna of Austria. Sebastian succeeded to the throne at the age of three, on the death of King John III, his paternal grandfather. Soon after his birth, his mother Joanna of Spain left her infant son to serve as Regent of Spain for her father, Emperor Charles V. After his abdication in 1556, she served in the same capacity for her brother Philip II of Spain. Joanna remained in Spain until her death in 1573, never to see her son again. Since Sebastian was still a child, the regency was handled first by his paternal grandmother, Catherine of Austria, and then by his great-uncle, Cardinal Henry of Évora. During this period Portugal continued colonial expansion in Angola, Mozambique, and Malacca, as well as the annexation of Macau in 1557.