At the time of his birth, Diego's elder brother,
Prince Ferdinand, was still the
heir-apparent. On the death of Ferdinand in 1578, Diego became heir-apparent to the throne. He was formally invested as
Prince of Asturias on 1 March 1580 by the Courts in Madrid. The poet
Cristóbal de Virués dedicated a sonnet to the new Prince, where he proposed that Diego follow the steps of his father. In 1580, his father became
king of Portugal as well, making Diego heir-apparent of that realm also. His mother, Anna, died during a trip to their new kingdom. Diego and his siblings remained in Madrid under the care of their half-sisters
Isabella Clara Eugenia and
Catherine Michelle. The letters of Philip II make clear that he was extremely proud of Diego: he wrote that his son had already learned the alphabet and dancing by the age of five. In a letter dated 1582 the king wrote to the Indian Viceroy
Francisco de Mascarenhas commanding that he bring an elephant to the Prince of Asturias as a gift. Philip II taught his son the Portuguese language so that one day Diego could speak as king with his Portuguese subjects. In addition, Philip II planned to betroth Diego to one of the daughters of
John, Duke of Braganza and
Catherine of Portugal, thus
strengthening his claim to the Portuguese throne. == Death ==