The barons were the king's tenants-in-chief, standing just below the Crown in the hierarchy. In return for their extensive land-holdings, the barons owed military service to the king, although they often paid a levy called scutage in place of direct military action. The barons also owed the king numerous customary payments relating to their estates. King John repeatedly disregarded customary practice by setting these payments at very high levels, seizing baronial lands and abusing the justice system. Many barons rebelled, although a minority remained loyal to the king.
On 17 May 1215 the rebel barons captured London and the following month they finally coerced King John into granting Magna Carta.
The barons were not revolutionaries. They wanted to safeguard their estates by restoring traditional feudal relationships and limiting the king's power to exploit his rights.
This limitation of royal authority through a written grant was their most radical and enduring achievement. It established the principle that the king was subject to and not above the law.