Prince Affonso of Portugal was born in May 18 1475 in Lisbon, Portugal, and died in a horse riding accident in the margins of the Tagus river on July 13 1491, 16 years old.
Afonso was the only son and heir of king John II of Portugal by his marriage with Leonor of Viseu, princess of Portugal. The king was very fond of him and named the smaller island of São Tomé and Príncipe after him (Príncipe meaning Prince in Portuguese language).
As a boy, Afonso was married to princess Isabella of Aragon, the eldest daughter of the catholic royal couple, reyes catolicos. Isabella I of Castile and Ferdinand II of Aragon had a male heir - Juan prince of the Asturias - but he was a feeble child and not expected to survive. Princess Isabella was thus the probable heiress to the joint throne of Castile and Aragon and, by being married to the heir of Portugal that meant a union of the Iberian kingdoms in Portuguese hands. The Spanish monarchs apparently desired this not to happen. They tried every diplomatic channel to have the wedding dissolved, without success, due to Portuguese influence with the Pope. Their cause was apparently lost, when an accident saved Aragon and Castile from future annexation.
Afonso died from a horse fall in mysterious circumstances, during a ride in the Tagus. Murder was never proved but the Spaniards had much to gain from his disappearence. Moreover, the prince's valet, an exiled Castilian boy, disappeared in the same day after being the only eyewitness of the event.
After his death, John II was left with no legitimate male heirs and the crown then went to Manuel I of Portugal in 1495, when king John died. Afonso's widow Isabella was married to Manuel, thus again creating the possibility of union of all Iberian kingdoms. Isabella bore a son, Miguel (born 1498 and died 1500). Manuel's possibilities to become the king-consort of Spain vanished by the death of Isabella in 1498. After the death of prince Juan of the Asturias, baby Miguel of Portugal became heir to the Spanish crowns and was recognized as Prince of the Asturias. His death 1500 put the final end to possibility of this union, and Joan the Mad (Miguel's aunt) became the heiress of Spain, eventually bringing those kingdoms to the Hapsburgs. The union took place about a hundred years later, 1580, when the Hapsburg king of Spain, Philip II, grandson of Joan the Mad and grandson of Manuel of Portugal, annexed Portugal and its dominions.