The Second Barons' War (1264–1267) was a civil war in England between the forces of a number of rebellious barons lead by Simon de Montfort, against the Royalist forces led by Prince Edward (later Edward I of England).

The reign of Henry III is most remembered for this period of civil strife which was provoked ostensibly by Henry III's demands for extra finances, but marked a more general disatisfaction 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 had originally been one of the foreign upstarts so loathed by many as Henry's foreign councillors, but after he married Henry’s sister Eleanor without consulting Henry, 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 in Sicily on behalf of the Pope in return for a title for his second son Edmund, a state of affairs which made many barons fearful that Henry was following in the footsteps of his father and needed to be kept in check, just as King John had. De Montfort became leader of those who wanted to reassert Magna Carta and force the king to surrender more power to the baronial council. In 1258 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 fifteen barons to deal with the business of government and providing for a three yearly meeting of parliament to monitor their performance.

Henry was forced to take part in the swearing of a collective oath to the Provisions of Oxford. In the following years, those supporting de Montfort 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. Civil War followed.

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 representation to include each county of England and many important towns – i.e. to groups beyond the nobility. 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. Following this victory savage retribution was exacted on the rebels and authority was restored to King Henry.

See also

• Henry III of England

• Simon de Montfort

• First Barons' War