Alfonso of Aragon, and I. of Naples and Sicily, surnamed the Magnanimous, born about 1390, died June 27, 1458. He succeeded his father Ferdinand I. in 1416, and the first act of his reign displayed the generosity of his character. Having received a list of nobles who were conspiring to dethrone him, he tore the paper in pieces without reading it. In the early part of his reign he left Spain to make good his claims to the sovereignty of the islands of Sardinia and Corsica, which were then partly in the power of the Genoese. In the war which followed he met with some success, but soon relinquished this project for more dazzling schemes of ambition. Joanna, queen of Naples, being attacked by Louis III., duke of Anjou, sent to Alfonso, offering to make him duke of Calabria and heir to the throne of Naples if he would aid her against the duke of Anjou. Alfonso eagerly accepted this proposition, abandoned Sardinia and Corsica, over which his sovereignty thenceforth amounted to but little, and, sailing to Naples, obliged the duke of Anjou to raise the siege, and make a peace on terms advantageous to the queen. But Joanna became jealous of the power of her new ally, and open war broke out between them.

The queen summoned to her aid Sforza Attendolo, the general of the duke of Anjou, who defeated Alfonso. The latter was soon enabled by the arrival of fresh troops from Spain to make himself master of the city of Naples, and to hold his enemies in check. But his presence was now required in Spain to protect his kingdom of Aragon, then at war with Castile. Accordingly, leaving his brother Don Pedro in charge of his affairs in Italy, he sailed for Spain in 1423. On his way thither he made a descent on Marseilles, then belonging to the duke of Anjou, captured the city without difficulty, but neither sacked it nor carried away from it any booty, with the exception of the body of a dead saint, Louis, formerly bishop of Toulouse. Alfonso passed about eight years in Spain, and then again turned his attention to Italy. Here the Spaniards, pressed by the queen, the pope, the dukes of Anjou and Milan, and the Genoese, had been almost overwhelmed. Alfonso arrived in 1432, and, seeing the desperate state of affairs, sailed to the island of Jerba on the coast of Africa, which he conquered, after gaining a victory over the bey of Tunis, to whom the island belonged.

After this exploit he returned to Italy, where he engaged in negotiations to bring about a reconciliation with Queen Joanna, and in intrigues to obtain adherents. In 1435 the queen died, bequeathing her crown to Rene of Anjou, count of Provence, brother and successor of Louis III., who had died some time before; and Alfonso, thinking the occasion a favorable one for asserting his claims, renewed the war, and besieged the city of Gaeta by sea and land. But in a naval battle near the island of Ponza, he was totally defeated by the Genoese and the duke of Milan, and was taken prisoner with a great number of his followers; and shortly afterward his land forces were routed and dispersed under the walls of Gaeta. Having by his nobleness of disposition and gallant bearing gained the affection of his captor, the duke of Milan, the latter set him at liberty and became his ally, and Alfonso was thus enabled to resume his operations under better auspices. After a contest of several years without effecting much, he succeeded, by the treachery of one of the adherents of Rene, in making himself master of Naples in 1442, and compelled Rene to seek refuge in Provence. Alfonso was soon after recognized as king of Naples by the assembled states of the kingdom, and by Pope Eugenius IV., who also issued a bull legitimatizing Ferdinand, the bastard son of the king.

From this time Alfonso resided in Naples, exerting himself to improve the condition of that kingdom, the affairs of which, during the reign of Joanna II. and the disturbances which followed, had fallen into much disorder; and, though taking part in some Italian wars of little importance, he passed the remainder of his life in comparative quiet. At his death his brother John inherited the crowns of Aragon, Sardinia, and Sicily, while his son Ferdinand received that of Naples.