Although many boys and young men from royal families have been felled by illness and bad luck there aren’t many who can be said to have been potentially murdered on their father’s orders.

Enter Don Carlos, Prince of Asturias.

The young prince Carlos was the son of Philip, Prince of Asturias, who would be King Philip II of Spain, and his first wife (and double first cousin) Maria Manuela, a princess of Portugal by birth. Philip and Maria’s sex life had been severely hampered on the orders of Philip’s father Charles, but Maria still fell pregnant just over a year after their marriage. The birth of a son was a moment of joy for Philip but it was soon tempered with sadness. Maria had been in labour for three days before Carlos had finally been born and four days after the birth in July 1545 she died, reportedly from a postnatal haemorrhage. Philip was left holding the baby.

But when you’re the heir to the throne you don’t actually need to be holding the baby yourself. There are servants and courtiers to do that for you. Carlos was left in the hands of said servants at the age of three when his father left Spain for the Netherlands, Germany and Italy. By the time he came back the boy was six years old. His aunt Maria and her husband Maximilian had been left in charge of Spain in her brother’s absence and after they left his aunt Juana took over managing Spain and her nephew so he had some family around to ensure he was being raised properly. But there was a definite absence of both mother and father during the boy’s infancy, let alone siblings to join him in the nursery, which must have made his childhood lonely.

In 1554 when he was nine years old Carlos gained his first stepmother, Queen Mary of England. However, Carlos never travelled to England to meet her and Mary never visited Spain. Her influence on his upbringing must have been minimal. She and Philip tried to conceive their own son to rule England but when Mary died in 1558 she was childless. Carlos was still Philip’s only heir. And the matter became a little more pressing in September 1558 when Carlos’ grandfather Emperor Charles V passed away. He had abdicated two years earlier and retired to a monastery. But his death meant that the line of succession was now solely on Carlos’ shoulders.

And those shoulders were not particularly broad. Carlos was not a strong child but he survived infancy although the odds were stacked against him. Geoffrey Parker has suggested that his mother’s protracted labour may have left him starved of oxygen at birth. Even without this Carlos was not particularly healthy, both his shoulders and legs were uneven and after contracting malaria he frequently came down with severe fevers. Some of his health problems may have stemmed from his poor gene pool - he was quite inbred. His parents had been double first cousins and on both sides of his family there were multiple instances of the Portuguese, Spanish and Burgundian royal families intermarrying over the generations.

In 1560 the Spanish royal family and its courtiers were gathered in Toledo to witness a grand event. Led by Carlos’ aunt and uncle, Juana and Juan, the attendees swore allegiance to Carlos and officially recognised him as heir to the Spanish throne once Philip died. The event was followed by the usual celebrations of banquets and jousts. It was the first step towards the prince being formally identified as next in line, clearly displaying the line of succession to the public. It also suggested that there was hope that now he was past the dangerous period of infancy he would survive to adulthood and one day become King Carlos II of Spain.

However, it nearly all fell apart. In 1562 the Prince slipped as he went down a flight of stairs and cracked his head open on a door. His doctors bandaged the injury but after ten days the wound became infected. At one point his father and doctors were convinced he was going to die and it was only when the remains of a “saintly” friar were brought in that he showed signs of recovery. In the following weeks he remained pale and physically weak. Some historians have suggested that this head injury was the cause of his later mental health problems, but others believe that even without this he would have developed issues in later life. His father had been absent and every motherly figure in his life either left for their husbands lands or died. It would have left him with an attachment disorder at the very least.

A step towards the throne and towards being recognised as an adult would inevitably lead to discussions around marriage. For a number of years Philip had been negotiating with the French court for Carlos to marry Princess Elizabeth, the eldest daughter of King Henry II and Catherine de Medici. Changes in policy in both countries meant that the match had been called off several times before being picked up again at a later date. But in 1560 when the French bride finally arrived it was for a different groom. Philip had remained unmarried since Mary’s death and ultimately had decided that Elizabeth would make just as good a wife for himself, despite her being just fourteen years old and nearly two decades his junior.

But the prince had to marry someone and over the following years more negotiations were held. Proposed brides included Mary Queen of Scots, a marriage with Spain would help keep her country Catholic and could also help her claim the English throne and bring England back into the Catholic fold. Then there was his stepmother’s younger sister, Margaret of Valois, who ultimately became Queen of Navarre. A third candidate was Carlos’ first cousin Anna of Austria who was the daughter of Philip’s sister Maria and their cousin Maximilian.

In 1564 the Austrians sent over an ambassador to negotiate the details of Anna’s betrothal to Carlos. The ambassador’s reports home give us some key details of the man that Carlos was becoming. According to the gossip the ambassador was able to pick up, the prince suffered frequent mood swings and had a violent temper. He reportedly asked lots of questions but didn’t always appear to understand the answers, and he may have had a speech impediment as his “r” and “l”s were often mixed up. However, on meeting the Prince himself the ambassador reported that Carlos seemed very sensible, had an excellent memory, and although he was quick to anger he did not appear to be malicious. After the betrothal was confirmed the prince hired a tutor in German and bought at least one book of German literature. Anna’s mother had raised her speaking Spanish but Carlos hoped to be able to converse with his wife in her native tongue as well as his own.

At the age of nineteen it seemed that despite Carlos’ temper and poor health there was still a bright future for him. His father had tried to get the court at Aragon to swear allegiance to him in 1563, as the Castilians had done in 1560. But Carlos’ health meant that he couldn’t travel with his father and the Aragonese courtiers refused to swear allegiance to a prince in absentia. Despite this setback Philip arranged for Carlos to attend various councils when he could not join them himself, giving his son a chance to experience government and policy making in person.

But in January 1568 everything fell apart for Carlos. His marriage to Anna was still not finalised and he may have become obsessed with the idea of travelling to Austria to claim her hand like a chivalrous knight from a romance. On the 16th January he summoned his uncle Juan, Philip’s illegitimate half-brother and admiral of the Spanish fleet, and begged him to provide a ship to Italy. Juan told Philip, who was still not set on the marriage, and when Juan returned to Carlos to discuss it further the prince tried to stab his uncle.

Carlos had been violent towards servants but threatening the life of a family member was a step too far to Philip. It suggested that his son’s behaviour was starting to escalate. Perhaps he was concerned that Carlos might kill him too in a fit of rage, or threaten the lives of the Queen and their children. Elisabeth had given birth to two daughters but who knew what Carlos might do if a healthy son was delivered in the future? It wasn’t a risk worth taking. Two days later, on the night of the 18th, Philip led a specially selected group of courtiers to Carlos’ chambers and had his son placed under house arrest.

Various papers were seized from Carlos’ chambers, and an audit of his accounts was carried out which showed that he was massively in debt. Although there were some questions over whether he was plotting against his father there doesn’t appear to have been any firm proof. Carlos appears to have written down almost anything that came to mind and it may have been this that convinced Philip that his son was too mentally unstable to make a suitable king.

The prince was kept firmly under lock and key. After dissolving his household, Philip had Carlos moved to a windowless apartment in the Alcazar at Madrid. The lack of windows may have been to stop his son shouting out for help, or even attempting to throw himself out of one. It appears that Philip hoped to keep his son alive but imprisoned and barred from the throne, as his own grandmother Juana of Castile had been by her father Ferdinand and then later by her son Charles. Juana had lived a long life and had been kept in relative comfort, perhaps Philip hoped the same could be done for his son. Elizabeth hadn’t given him a son yet but it was surely only a matter of time before a healthier prince came along.

But Carlos didn’t want to play along. Reports suggest that he went on hunger strike and refused to eat or drink until his jailers force fed him. He also swallowed a diamond believing that this would poison him. As time passed his jailers appear to have given up on the idea of force feeding the young man, perhaps because it was too difficult to do without fear of seriously hurting him.

On 24th July 1568 Carlos died at the age of 23. The letters from his jailers show that he refused to eat for over two weeks and by the time he changed his mind he was no longer able to stomach food. He did, however, drink copious amounts of ice water, which Geoffrey Parker suggests was a sign that his malaria infection had flared up again. The combination of starvation and the virus probably killed him. But a few sources claimed that the prince had been poisoned on his father’s orders in order to clear the line of succession for a future, more promising son.

It’s unlikely that Philip had ordered his son's death. After all he was still without a legitimate male heir and history is riddled with the civil wars that break out when there’s no obvious direct line of inheritance. If the malaria didn’t finish him off then Carlos would have succeeded in killing himself through starvation. It was a sad end for a man who had been so alone as a child.

The last "Almost King" was William, Duke of Gloucester.