King John had absolutely no intention of following through on the commitments he made in the Magna Carta. As far as he was concerned, signing the document was just a delaying tactic, an opportunity to buy him some time before the ongoing armed conflict with his restive barons would break out once again. On the face of it, the provisions of the Magna Carta were quite radical for their time, especially those clauses that subjected the king to the law of the land for the very first time in English history.
The problem, however, was that there was no enforcement mechanism in place to hold John to his promises. After all, the king was regarded as the fount and origin of justice. The law courts were the king's courts just and the judges were the king's judges. So John could put his signature to the Magna Carta safe in the knowledge that he still had the full legal apparatus of the state in his hands, not to mention a string of well-fortified castles and other strategic strongholds that would allow him to take the fight to his opponents. And so it proved. In less than three months, the Magna Carta was a dead letter, and John was once more at war with his barons.