Second Barons ' War The Second Barons ' War ( 1264 – 1267 ) was a civil war in England between the forces of a number of barons led by Simon de Montfort , against the Royalist forces led by Prince Edward ( later Edward I of England ) , in the name of Henry III . Causes . The reign of Henry III is most remembered for the constitutional crisis in this period of civil strife , which was provoked ostensibly by Henry III ' s demands for extra finances , but which marked a more general dissatisfaction with Henry ' s methods of government on the part of the English barons , discontent which was exacerbated at a more popular level by widespread famine . French - born Simon de Montfort had originally been one of the foreign upstarts so loathed by many lords as Henry ' s " foreign councillors " , but having inherited through his mother the English title Earl of Leicester , he married Henry ’ s sister Eleanor with Henry ' s permission , but without the agreement of the English Barons , since it was a matter of the state : a feud developed between the two . Their relationship reached a crisis in the 1250s , when de Montfort was put on trial for actions he took as lieutenant of Gascony , the last remaining Plantagenet lands across the English Channel . Henry also became embroiled in funding a war against the Hohenstaufen in Sicily on behalf of Pope Innocent IV in return for the Hohenstaufen title King of Sicily for his second son Edmund . This made many barons fearful that Henry was following in the footsteps of his father King John and , like him , needed to be kept in check . When Henry ' s treasury ran dry , Innocent withdrew the title , and in regranting it to Charles of Anjou in effect negated the sale . Simon , earl of Leicester , became leader of those who wanted to reassert the Magna Carta and force the king to surrender more power to the baronial council . In 1258 , initiating the move toward reform , seven leading barons forced Henry to agree to the Provisions of Oxford , which effectively abolished the absolutist Anglo - Norman monarchy , giving power to a council of twenty - four barons to deal with the business of government and providing for a great council in the form of a parliament every three years , to monitor their performance . Henry was forced to take part in the swearing of a collective oath to uphold the Provisions of Oxford . In the following years , those supporting de Montfort , including his circle of Franciscan advisors centered on Adam Marsh , and those loyal to the king grew more and more polarised ; Henry obtained a papal bull in 1261 exempting him from his oath , and both sides began to raise armies , the Royalists under Edward Longshanks , Henry ' s eldest son . A civil war followed . Course of the war . The charismatic de Montfort and his forces had captured most of southeastern England by 1263 and at the Battle of Lewes in 1264 , Henry was defeated and taken prisoner by de Montfort ' s army . While Henry was reduced to a figurehead king , de Montfort broadened parliamentary representation to include groups beyond the nobility , members from each county of England and many important towns . Henry and Edward continued under house arrest . The short period which followed was the closest England was to come to complete abolition of the monarchy until the Commonwealth period of 1649 – 1660 , and many of the barons who had initially supported de Montfort began to suspect that he had gone too far with his reforming zeal . Only fifteen months later de Montfort ' s gains were reversed when Edward Longshanks escaped captivity to lead the royalists into battle again , defeating and killing de Montfort at the Battle of Evesham in 1265 . His son , Simon , attempted a negotiated surrender but it was rejected by de Montfort loyalists . The impasse culminated in the six - month Siege of Kenilworth at which the King prevailed . De Montfort ' s forces were permitted to leave the castle with their weapons and horses . Following this victory , savage retribution was exacted on the rebels and authority was restored to King Henry . The casualties of the war are estimated at 15 , 000 .