John I the Posthumous was the King of France for five days, from the time he was born until the time he died.
Male-preference primogeniture – under which the oldest son of a ruler becomes the new ruler on his father’s death – ran into an unusual complication in 1316. Louis X, the King of France, died with no male heir… but he left behind one daughter and one pregnant wife. Who was the new ruler?
Louis’ daughter Joan was four years old and France had carefully avoided having any female monarchs for a long time. If Louis’ newer child was born a boy then he would be the old king’s first-born son… but that child was not due to arrive for another five months. And no-one knew if this kid would even be a boy – and therefore the “rightful” king.
The technical term for children born after the death of one of their parents is “posthumous birth,” and there have been several examples of this phenomenon throughout history: Muhammed, the playwright Ben Jonson, Isaac Newton, Jonathan Swift, Saddam Hussein, and three Presidents of the United States. In the case of Louis X, his brother Philip the Tall took over as regent until the posthumous child arrived to claim the throne.
John I was King of France from the moment he was born. He died just five days later. We have no idea what killed him. In this time newborn children died for any number of reasons, and vulnerable kings also died for many nefarious reasons, so your guess is as good as mine. Rumours abounded that he had been murdered, or even that he had been spirited away and was still alive somewhere, ready to retake the throne.
What followed was a power struggle between the regent Philip the Tall and supporters of the infant princess Joan. Philip won the fight and was crowned as Philip V of France; Joan went on to marry her cousin and become ruler of Navarre. Philip died without children and the French throne went to his younger brother Charles… who also died leaving a pregnant wife behind. Another regency followed, led by another Philip, while the kingdom waited… but King Charles’ wife gave birth to a daughter, finally ending that line of succession.