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 by widespread famine.
French-born Simon de Montfort, Earl of Leicester, 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 without Henry's permission, and without the agreement of the English Barons (ordinarily necessary since it was a matter of state). As a result, a feud developed between de Montfort and Henry. 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 Dynasty 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 de Montfort 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.
Seeking to restore his position, in 1259 Henry purchased the support of King Louis IX of France by the Treaty of Paris, agreeing to accept the loss of the lands in France that had been seized from him and from his father King John by Louis and his predecessors since 1202, and to do homage for those that remained in his hands. In 1261 he obtained a papal bull releasing him from his oath, and set about reasserting his control of government. The baronial opposition responded by summoning their own Parliament and contesting control of local government, but with civil war looming they backed down and de Montfort fled to France, while the other key opposition leader, Richard de Clare, Earl of Hertford and Gloucester, switched over to the King's side. Under the Treaty of Kingston an arbitration system was agreed to resolve outstanding disputes between Henry and the barons, with de Clare as the initial arbiter and the option of appealing his verdicts to Louis IX. However, continued Poitevin influence and the failures and renewal of provocative policies by Henry's government soon inflamed hostility once more. The King's position was further weakened by the death of Richard de Clare and the succession of his son Gilbert, who sided with the opposition, and by the reversal of the papal annulment of his oath to uphold the Provisions.