Concerned over King John’s warlike preparations and Papal support, the Baron’s agreed to muster at Stamford near Peterborough during Easter week (19-26 April) 1215. Five Earls and forty Barons, mostly from the north, are mentioned by name as being present at the muster,” with many others“; they all came with horses and arms, and brought with them ” a countless host,” estimated to comprise about two thousand knights, besides other horsemen, sergeants-at-arms, and foot soldiers.
From Stamford they marched to Northampton, John, who had spent Easter in London, sent William Marshal, Archbishop of Canterbury, Steven Langton and some other bishops and magnates to parley with them. Several meetings appear to have taken place. At some point the Baron’s moved to Brackley. It was here they met with Marshal and the Archbishop on 27 April, and “presented to the envoys a certain schedule, which consisted for the most part of ancient laws and customs of the realm, declaring that if the king did not at once grant these things and confirm them with his seal, they would compel him by force.” It was this list of demands that became the Magna Carta.
Langton and the Marshal returned to the king, now in Wiltshire, with the list of demands. One by one the articles were read out to him by the Archbishop. According to Roger of Wendover, after he had heard them all John said ” Why do these barons not ask for my kingdom at once ?… their demands are idle dreams, without a shadow of reason” Then he burst into a fit of rage, and swore that he would never grant to them liberties which would make himself a slave.
He then sent the two back to the Barons and instructed them to repeat his words verbatim. On hearing this, the Barons immediately renounced their fealty to the King and chose Robert Fitz-Walter as their leader, to whom they gave the grandiose title of ” Marshal of the army of God and Holy Church.” They then marched back to Northampton, occupied the town and laid siege to the castle. However, they had not brought any siege equipment and after two weeks, were forced to give up, but not before many had been killed including Fitz-Walters standard bearer. The Barons moved to Bedford where William de Beauchamp readily gave up his castle and then to London.
A truce was brokered between the two sides but by the beginning of June it had ended. On 29 May, John wrote to the Pope, complaining about the barons, and how it was impossible for him to fulfil his vow of a crusade. In reality, John knew it was all over. The king was almost deserted and at one point only had seven knights left. Four rebel armies were now in the field, one besieging the Tower of London, another capturing Lincoln, with none of them paying any attention to the truce. The townsfolk of Northampton had also risen against the royal garrison of
the castle and slain several of them. Although the King’s men had sallied out and burnt half the town. Finally, sometime at the beginning of June, John despatched William Marshal to the Barons in London saying “…that for the sake of peace and for the welfare and honour of his realm, he would freely concede to them the laws and liberties which they asked ; and that they might appoint a place and day for him and them to meet, for the settlement of all these things.” In the words of Ralph of Coggeshall, ” By the intervention of the archbishop of Canterbury, with several of his fellow-bishops and some barons, a sort of peace was made.” A meeting was arranged where John would agree to the Barons demands. The meeting was set for 15 June, and the place, a meadow between Staines and Windsor called Runnemede.