It is a common misconception that King John allowed the barons to have more power by signing the Magna Carta.
Although the Magna Carta was signed by King John in 1215, the agreement lasted little more than three months. It would not properly come into affect until after King John’s death.
Whilst King John was signing the Magna Carta to appease the nobles, he was corresponding with Pope Innocent III in Rome. During this correspondence, he informed the pope that the charter clashed with the terms of an agreement King John and Innocent had signed a few years earlier and that it took power away from the clergy in England.
The Pope’s reaction was to excommunicate all the barons and declare the Magna Carta to be an illegal and immoral document.
This led to a civil war in England that lasted long past King John’s death in 1216. Eventually the Magna Carta was reintroduced during the reign of John’s young son, King Henry III.