By 1215 King John had lost almost all of his northern continental possessions. The core of the Angevin empire, Normandy, was lost. Anglo-French barons who still held lands in Normandy owed their primary allegiance to King Phillip Augustus, not to King John. The barons and churchmen who remained under his sovereignty chaffed under his rule. It is clear from the document that the barons forced John to sign when they met with John on Runnymede in 15 July 2015, they intended to impose reform on the king. We might sum up their objectives as being the administration of justice and defending their customary rights in what remained of the John’s kingdom.
Kenneth Pennington, Reform in 1215: Magna Carta and the Fourth Lateran Council, 32 BULL. MEDIEVAL CANON L. 97 (2015).