King John’s death was vital for the survival of Magna Carta, if he’d won the war there would probably be no charter at all. But the minority government of his nine-year-old son, to win the war, to win the hearts and minds of all the people fighting against them, decided that they would issue a new version of the charter.
After 1215 Magna Carta should, in theory, just have died. It was part of a peace settlement, the peace settlement failed, the barons went back to war against the king, the king went to war against the barons, England was invaded by the French. That should be the end of it. But, it contained principles that could still be used for negotiations between King John’s nine year old son, Henry III, who succeeded in 1216, and the barons who were still at war against him. And therefore it was reissued just over a year later in November 1216. And then it was reissued again at the end of that civil war in 1217 and then again in 1225 as an attempt to negotiate taxation between the English Church and the king. Thereafter it was reissued regularly whenever there were problems between the realm and its sovereign and by the end of the 13th century it had become totemic. In the 1270s we find the Church demanding that a copy of Magna Carta be displayed on the door of every major monastery and every cathedral church. It had become a totem, not just a legal settlement.
Magna Carta was very important for the whole development of parliament. First of all it asserted a fundamental principle that taxation needed the consent of the kingdom. Secondly, it made taxation absolutely necessary for the king because it stopped up so many sources of revenue. And thirdly, in making concession to knights in the county, to burgesses in the towns, it looked forward to their representation in Parliament. They were going to have to be summoned to give consent to taxation. And so Magna Carta laid the foundations for the tax-based parliamentary state.
You could argue that Magna Carta made a major difference to English politics in the 13th century, certainly it cropped up again and again in political debate. You could argue that it prevented kings of England from, as it were, exploiting their rights to the absolute full. It put an end to arbitrary kingship. And yet, kings continued to tax, they continued to tyrannise their subjects for centuries thereafter. What mattered about Magna Carta, I think, was Magna Carta the idea, not necessarily Magna Carta the political tool. It survived long after the tyranny of any individual king and therefore it became a point of principle rather than of practical politics.