 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 ( the 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 . 
