The conflict between king and magnates was caused by dissatisfaction with the influence of foreigners at court, and Henry's high level of taxation.

In 1258 Henry was forced to accept the Provisions of Oxford that essentially left royal government in the hands of a council of magnates, but this document went through a long series of revocations and reinstatements.

One source of discontent was the influence two groups of royal favourites enjoyed at court: the Savoyards, relatives of Queen Eleanor of Provence, and the king's half-brothers, known as Poitevins or Lusignans.

Secondly, the king had in 1254 accepted Pope Innocent IV's offer of the crown of Sicily for his younger son Edmund.

In 1258, Henry was forced to accept the so-called Provisions of Oxford, whereby he effectively surrendered control of royal government to a council of magnates.

[10] Cornered, Montfort had to accept a truce and agree to submit the issue to arbitration by the French king Louis IX.

[14] Referring to the papal writ of annulment, Henry asked the French king to free him from observing the provisions forced upon him by the barons.

[24] The settlement starts out by reiterating the declarations of the two parties, where they place the decision fully in the hands of the French king.

[30] Louis was a firm believer in the royal prerogative, and was never likely to embrace the precedents set by the barons' infringement of Henry's authority.

At the same time, Henry's wife Eleanor of Provence – who was Louis' sister-in-law – had worked hard to procure a favourable decision for her husband.

[13] Hostilities started already in February, when Montfort's sons, Henry and Simon the Younger, attacked the possessions of Roger Mortimer in the Marches.

[33] Henry summoned the feudal army, and the royal forces won an important victory at Northampton, where the younger Simon was captured.

When he set out in pursuit of the fleeing soldiers, however, he left the rest of the royal army open to attack by the baronial forces, who soon won the day.

He negotiated a treaty with Llywelyn ap Gruffudd, the Prince of Wales, an act that made him unpopular with the English Marcher lords.

[39] Edward, however, routed the younger Simon at Kenilworth Castle, and on 4 August 1265 Montfort found himself trapped at Evesham, forced to give battle with a much smaller army than the royals.

In October 1266 the Dictum of Kenilworth set down terms by which the rebels could obtain pardons, and by the end of the year the garrison surrendered.