Don Carlos (July 8 1545 – July 24 1568), Prince of Asturias was the son of King Philip II of Spain by his first wife Maria Manuela, daughter of John III of Portugal.

He was born at Valladolid, and his mother died a few days after his birth.

The young Infante Carlos was delicate and deformed. He grew up proud, wilful, and indolent, and soon began to show signs of insanity. No doubt many of his problems stemmed from the extreme inbreeding that was a chronic aspect of the House of Habsburg and the royal houses of Portugal and Spain. Carlos had only six great-great-grandparents, instead of the more normal 16, and two of his great-grandmothers were sisters. His namesake, King Carlos II of Spain, was also highly inbred and debilitated as a consequence.

In 1559, Prince Carlos was betrothed to Elizabeth of Valois, a daughter of King Henry II of France, but a few months later she became the third wife of his father. Other brides were then suggested for the prince; Mary Queen of Scots, Marguerite de Valois, another daughter of Henry II of France, and Anne of Austria, a daughter of Maximilian II, Holy Roman Emperor, who was to become his father's fourth wife. Meanwhile, Carlos's mental derangement became more acute, and his condition could no longer be kept secret.

Despite his many afflictions, in 1560 he was recognized as the heir to the throne of Castile, and three years later to that of Aragon.

In 1562, he met with an accident which was followed by a serious illness. After his recovery, he showed more obvious signs of insanity, while his conduct both in public and in private was extremely vicious and disorderly. He took a marked dislike to the Duke of Alva, possibly because he wished to proceed to the Netherlands instead of the duke. He also exhibited a morbid antipathy towards his father, whose murder he even contemplated. At length in January 1568, when he had made preparations for flight from Spain, he was placed in confinement by order of Philip, and in the same year he died mysteriously—possibly from poison.

The opera 'Don Carlos' by Giuseppe Verdi is based on the 1787 drama by Friedrich Schiller which is loosely based on the historical conflict between Carlos and his father. The opera was first performed in French in Paris in 1867. Later, an Italian version was prepared that is referred to as Don Carlo.

Abstracted from Enyclopedia Britannica 1911 and several other sources