King John, as anyone familiar with the tales of Robin Hood can tell you, was a very bad egg.
His brother and predecessor, Richard, was keen on war, and spent most of his time abroad fighting. That was expensive. But at least he tended to win.
John, on the other hand, was a terrible military chief. But he still fought many wars in France. To pay for them, he taxed his nobles heavily – he was king, after all. And what he said went. The barons just had to pay up.
Add to that his generally outrageous attitude to justice – imposing hefty charges, taking hostages, and carrying out ruthless punishments – plus his battles with the Church, and things eventually came to a head. You can’t keep squeezing your lemons without expecting the pips to squeak.
The barons had had enough. They rebelled. Led by Robert fitz Walter, himself hardly a saint, they renounced their fealty to the king and captured London on 17 May 1215. And on 15 June, 1215, they met the king at Runnymede, by the Thames in Surrey, and demanded he sign a peace treaty – Magna Carta.
John agreed, and for the first time the principle was established that the king was subject to the laws of the land. The agreement also guaranteed certain rights to nobles and ‘free men’. (Your average serf, however – the vast majority of England’s population – still had virtually no rights, and the principles contained in Magna Carta did not apply to them.)
The document was rewritten within ten years, and most of its original 63 clauses are now irrelevant. But some, including the right to a fair trial by one’s peers, and the principle that taxes will not be levied without the “consent of the realm”, have formed the foundations of many a constitution and important document, including the United States Bill of Rights and the Universal Declaration of Human Rights.