The nobility took advantage of the minority of Henry III (1390-1406) to pursue their own gain at royal expense, but once the king reached adult age he strongly asserted his power. Royal prestige and authority suffered terribly during the long reign of his son, John II (1406-54). The king's uncle, Fernando de Antequera, who acted as regent, maintained stability until he was elected king of Aragon in 1412. John II, a disinterested monarch, allowed Alvaro de Luna, the royal favourite, to dominate him and to direct royal policy. Fernando's sons, Henry and John of Navarre, tried to gain control of the king and the organs of government, but Alvaro successfully thwarted their schemes. Alvaro retained effective authority during most of the reign, but in 1453 he incurred the king's wrath and was summarily executed.
The nobles continued to engage in an intense struggle for influence and power in the reign of Henry IV (1454-74). Although Juan Pacheco, Marqués de Villena, initially gained ascendancy over the king, others vied for royal favour. The nobles, alleging Henry's impotence, refused to accept the legitimacy of the infanta Joan, who, they declared, was the child of the queen and the king's most recent favourite, Beltrán de la Cueva. On that account the young girl was derided as La Beltraneja. Henry IV repudiated her and recognized his sister Isabella as heir to the throne in the Pact of Los Toros de Guisando in 1468. Although Villena and his supporters hoped to control Isabella, they soon learned that they could not. In 1469, without first seeking her brother's consent, as she had promised, Isabella married Ferdinand, son and heir of John II of Aragon. An angry Henry IV denounced her and tried to exclude her from the succession, but when he died, she was proclaimed Queen Isabella I (1474-1504). Alfonso V of Portugal, who was betrothed to Joan (La Beltraneja), invaded Castile on Joan's behalf, but in 1479 she abandoned her rights to the throne. Ferdinand's accession to the Aragonese throne in the same year brought about a personal union of Aragon and Castile.
John I (1387-95) acknowledged the pope of Avignon. Both John and his younger brother and successor, Martin I (1395-1410), had to attend constantly to agitation and unrest in Sardinia and Sicily. When Martin died without immediate heirs, the Crown of Aragon faced an acute crisis. The Compromise of Caspe, announced in 1412, determined that Fernando de Antequera, brother of King Henry III of Castile, had the best claim to the throne by right of inheritance. The accession of Ferdinand I (1412-16), the first of the Trastámara dynasty to rule in Aragon, prepared the way for the eventual union of Aragon and Castile. By withdrawing obedience from the Avignonese pope Benedict XIII, Ferdinand helped to terminate the schism. Alfonso V the Magnanimous (1416-58) opted to pursue ambitions in Italy and generally neglected his peninsular domains. After occupying the kingdom of Naples in 1442, he hoped to lord it over the rest of Italy and to extend his influence and power into the eastern Mediterranean. A spirit of discontent fostered by his long absence from Aragon provoked a crisis during the reign of his brother, John II (1458-79). John inherited the mainland kingdoms as well as Sicily, while Alfonso V's illegitimate son, Ferrante, obtained Naples. John had already added another kingdom to the Trastámara holdings when he married the queen of Navarre in 1420. By quarreling with his son, Prince Charles of Viana, he antagonized his people and provoked the open hostility of the Catalans. Many concluded that Charles had been poisoned when he died suddenly in 1461. Already restive because of economic and social uncertainties, the Catalans revolted and offered the principality to other potential rulers. Louis XI of France seized the opportunity to occupy Roussillon and Cerdagne, thereby laying the foundation for future enmity between France and Spain. Though hard pressed on all sides, John II suppressed the Catalan revolt by 1472 and then aided his daughter-in-law Isabella in acquiring the Castilian crown. His son Ferdinand succeeded him as king of Aragon and Sardinia, and his daughter Eleanor inherited Navarre.
The union of Aragon, Valencia, and Catalonia under John II of Aragon was extended to association with Castile through the marriage of his son Ferdinand with the Castilian heiress Isabella. The alliance grew toward union after the accession of the two sovereigns to their thrones in 1479 and 1474, respectively, and with joint action against the Moors of Granada, the French in Italy, and the independent kingdom of Navarre. Yet, at the same time, provincial institutions long survived the dynastic union.
Castilian interest in the New World and Aragonese ties in Italy, moreover, resulted in the ambivalent nature of Spanish 16th-century policy, with its uneasy alternation between the Mediterranean and the Atlantic. The monarchy increased the central power by the absorption of military orders and the adaptation of the Hermandad, or police organization, and the Inquisition for political purposes. During the reign of Charles I (the emperor Charles V) centralization was quickened by the importation of Burgundian conciliar methods of government, and in the reign of his son Philip II Spain was in practice an autocracy.
During the reign of John II, a patron of poets and scholars, the Italian Renaissance began to influence Castilian writers and thinkers, including the Marqués de Santillana (d. 1458), whose lyric poems have great beauty; Jorge Manrique (d. 1479), who reflected on the vanities of the world in the Coplas por la muerte de su padre (Verses on the Death of His Father); and Fernán Pérez de Guzmán (d. c. 1460), who sketched the characters and personalities in the court of Henry III in the Generaciones y semblanzas (Generations and Sketches).
In Aragon, Lo Crestià, an encyclopaedic work dealing with moral and political theory, was the best-known work of Francesc Eiximenis (d. 1409). The poet Ausiàs March (d. 1459) explored the psychological dimensions of love.
In 1450 the university of Barcelona was founded. Printing had its beginnings in Valencia and Barcelona in 1474 and in Lisbon in 1489.