In 1256, Simon de Montfort, earl of Leicester, assumed the leadership of English barons determined to halt Henry III’s misrule. In 1258, at the Council of Oxford, under the additional threat from Welsh lords, they agreed to rule according to the provisions of his father’s Magna Carta. By 1263, following further troubles on the Welsh borders, opposition to Henry had fragmented. This would have an important effect on the way the subsequent war of revolt was fought. De Montfort returned from France to join the earl of Derby and Gilbert of Clare (heir to the earldom of Gloucester) as the only notable nobles in opposition to the king, together with the citizens of London. Most magnates, especially the warlike lords of the Welsh marches, supported Henry III.

Earl Simon’s threat to Rochester terminated the successful royalist chevauchee in the Midlands and drew Henry Ill’s army south. Simon united his forces in London, then advanced to provoke battle before his support melted away. He won the battle at Lewes, but the royal forces escaped into Lewes priory and subsequent negotiations let the marcher barons go free to fight again.

Feuding between rival nobles in the marches turned into civil war when Henry III unfurled the royal banner, a red silk dragon, on 3 April 1264. His army at Oxford, summoned on the pretext of invading north Wales, held a central position between the Montfortians in the Midlands and the south-east. Their strength lay in Kenilworth Castle, London, and Dover, described as the ‘key to England’ by a contemporary writer.

The royalists struck rapidly at Northampton. The town and castle, with a rich haul of prisoners, fell almost at once, rather than resisting long enough for de Montfort to bring relief. His town of Leicester quickly followed. Instead of pressing the advantage, prince Edward indulged himself by attacking Derby’s estates and the castle of Tutbury. This gave de Montfort the opportunity to launch the manouevre which culminated in battle at Lewes.

By assaulting Rochester, de Montfort presumably meant to remove a threat to London. He crossed the Medway although the bridge was defended, and quickly took the town and the castle bailey (18 and 19 April). However, as in 1215, the great tower held out. After moving south at a leisurely speed, part of Henry’s army made a spurt to cover some 45 miles (72 km) on 26 April. If the aim was to surprise the Montfortians, it failed. News of Henry’s approach (night of 25-26 April) caused de Montfort to return to the security of London. His strategy had failed to capture Rochester, but by bringing the king south it permitted the Montfortian forces to unite in London. While Henry’s forces moved into Sussex prior to investing London or Dover, de Montfort marched south to seek battle. This was a risky strategy, but was the only way to seize the initiative after a run of reverses.

News of de Montfort’s movement caused the royalists to cancel their advance to Canterbury. Instead, they moved cautiously to Lewes, whose castle, town walls, and priory offered a secure base. De Montfort bivouacked at one of his own manors nearby. The royalists’ leaders, confident after their recent successes and outnumbering de Montfort’s army, rejected compromise. The size of the armies is unknown. Henry probably had over 1,000 cavalry – heavily armed knights and lighter esquires and sergeants – and de Montfort only one-third as many. Several thousand infantry were present, archers and spearmen from London, Kent, and Sussex, but probably not of high quality.

De Montfort had nothing to gain from delay and a great deal to lose. He surprised the royal army by seizing the high ground west of Lewes early on 14 May 1264. Only a broad outline of the battle of Lewes can be given. It probably took place west of the town. De Montfort’s army advanced down the slope, suffering an early setback when its left, of cavalry and the Londoners, was routed by prince Edward and the marcher barons. Edward, inflamed by a personal quarrel with the citizens, pursued them for several miles. While he was absent with the most experienced soldiers, the battle was lost: the royalist centre and left were smashed. Henry, who had two horses killed under him, sought refuge in the walled priory south of Lewes. By midday the main battle was over. When Edward returned, he plundered de Montfort’s baggage before his force was scattered. He joined his father in the priory with the remnants of his division. The bulk of dead were the lightly armed infantry. A contemporary estimate of 2,000 dead has found support in the discovery of mass graves holding over 1,000 corpses. However, de Montfort had not won an outright victory. Royal forces held the castle and priory, and a long siege would be politically risky. A compromise peace (15 May) left Henry and Edward in de Montfort’s hands, but the marcher barons, including his avowed personal enemy Roger Mortimer, escaped.

By late May 1265, de Montfort’s position was deteriorating as Gilbert of Clare defected and prince Edward escaped. With the marcher barons they isolated de Montfort (at Hereford with few men) west of the Severn by seizing the bridges, planning to defeat him before he received help. De Montfort seriously underestimated the danger until late June, when he made for Newport in Wales, intending to reach England by ship. Monmouth and Usk castles delayed him, then his ships were intercepted, and finally Edward caught up with him at Newport. De Montfort burned the bridge and escaped to Hereford by crossing the inhospitable Black Mountains.

After earl Simon’s rapid marching allowed him to escape being trapped west of the Severn, Edward occupied a central position at Worcester. After a night march and feint north he ambushed a Montfortian force at Kenilworth, then divided his army to trap Simon, whose force was overwhelmed at Evesham before he could unite with his son.

After failing to crush de Montfort, Edward was recalled to the Severn by the approach of de Montfort’s son Simon, who had organized an army in London. In mid-July he sacked Winchester, then waited at Oxford, hoping to draw off Edward’s army or join his father. He reached Kenilworth on 31 July, and at once his father earl Simon set off by night march (1-2 August) to unite their armies. On the same night, Edward marched on Kenilworth, making a feint towards Shrewsbury, having learned that the Montfortians had camped outside the castle. At dawn, the royalists fell on the sleeping army and captured many, before returning to Worcester to finish off de Montfort.

On learning that his son still had an army, de Montfort made a final attempt to join him. On the night of 3-4 August, he marched south-east to put the Avon between his army and pursuit. At daybreak, he paused fatefully at Evesham, while his son rested 9 miles (14 km) distant at Alcester, both on the north bank of the Avon. By a remarkable feat of anticipation, Edward trapped earl Simon in Evesham, using his great advantage in numbers. Mortimer’s division, following de Montfort, blocked escape to the south back over the bridge; Clare held the road north to Alcester; while Edward overtook de Montfort to ensure the Kenilworth road south of the Avon was clear before rejoining Clare. When de Montfort caught Sight of Edward’s division he at first believed it to be his son’s.

Earl Simon and his followers had no choice but to fight. In the early morning, taking the captive king with them, Simon with his cavalry advanced to the flat hill-top north of Evesham. Their charge failed to pierce the royalist line of Edward and Clare, which then enveloped de Montfort’s force, crushing it in a dense mass. De Montfort’s cavalry, left in Evesham, escaped the ensuing massacre. The king, wearing borrowed armour, narrowly escaped. It was unusual for more than a handful of heavily protected and ransomable nobles to die in battle, but at Evesham at least thirty perished. De Montfort, unhorsed, was deliberately killed, and his body horribly mutilated.

Unlike Lewes, Evesham was decisive, even though Montfortian strongholds resisted until mid-1267. The major problem was Kenilworth. Behind its complex water defences the castle defied the resources of the kingdom for six months; bombardment of the walls and assault by barges brought from Chester both failed. Finally, starvation brought about its surrender in December 1266. If the two battles of 1264-65 showed little tactical ingenuity, the maneuvering which preceded them shows both de Montfort and Edward to have been skilled generals, possessing strategic oversight and capable of bold strokes. Both based their moves on sound information-gathering, a capacity with which medieval commanders have rarely been credited.