At the Battle of Lewes in 1264, the rebellious barons, led by Simon de Montfort, had defeated the royal army and taken King Henry III captive. For the next year, the reins of government were in Montfort's hands, but his support soon began to crumble. On 4 August 1265, Montfort faced an army led by Prince Edward (later King Edward I) and the powerful Earl of Gloucester, who had recently defected to the royalist side, at the Battle of Evesham. The battle resulted in a complete royal victory; Montfort was killed, and King Henry III was restored to full power.
Part of the rebellious forces held out, however, and their stronghold was the virtually impregnable Kenilworth Castle. In the summer of 1266, a siege of the castle was initiated, but the effort proved futile. There were rumours that Montfort's son Simon the Younger was planning an invasion of England from Normandy, and this was the hope that the rebels hung on to. It was in this situation that the papal legate Ottobuono Fieschi exerted his influence, to make the king pursue a more conciliatory policy. In August, the king summoned a parliament at Kenilworth, where the siege was ongoing. He commissioned a number of earls, barons and bishops to draft a treaty of reconciliation.