Battle of Evesham
Such was the murder of Evesham, for battle it was none.
Background
After the Battle of Lewes, Simon de Montfort was in a dominant position in England. He held King Henry III, Prince Edward (Future King Edward I), and the King's brother, Richard of Cornwall, as prisoners.
However, his influence began to wane as key allies started to disappear. Robert de Ferrers, Earl of Derby, was arrested and imprisoned in the Tower of London, and most importantly, Gilbert de Clare, Earl of Gloucester, fell out with de Montfort and joined the royalists in the spring of 1265. Gilbert de Clare then assisted Prince Edward in escaping de Montfort's captivity in Hereford.
Prince Edward quickly began to amass forces to join the royalists. Simon de Montfort saw the impending forces gathering against him and left London for Wales, where he made a deal with Llywelyn ap Gruffudd, Prince of Wales, who provided spearmen in return for full recognition of his title and the promise that he could retain all his military gains.
As this was happening, Edward took control of the towns of Gloucester and Worcester and blocked all major crossings on the River Severn, isolating de Montfort and his forces in Wales.
Simon's goal was to unite with the forces under his son, Simon de Montfort the Younger, to then face Edward's army. Simon the Younger and his army were heading west and arrived at Kenilworth. This caused Edward to move his forces from Worcester east to try and engage Simon the Younger before he could unite with his father's army. When Edward's army moved, Simon de Montfort quickly moved his army into England and settled on a loop in the River Avon at Evesham to rest.
The morning of August 4, 1265, began with a thunderstorm accompanied by heavy rain. As people sought shelter from the rain in Evesham, Simon de Montfort received word from his scouts that an army was approaching from the north, flying the banners of his son, Simon de Montfort the Younger.
An hour later, the faint sound of a horn announcing the approaching army could be heard. Simon looked up in disbelief. An army of 10,000 strong was amassing to the north, but it was not his son's army; it was Prince Edward's army, forming a battleline on the elevated land outside of town. Edward had met Simon the Younger's forces at Kenilworth and defeated them, taking some of their banners to disguise his approach to Evesham and trick the scouts. Roger Mortimer of Wigmore split off from Edward's main force, and he and his men blocked the bridge at Bengeworth, to the south of Evesham, cutting off de Montfort's escape route.
After the final prayer before battle, Simon de Montfort's Standard-bearer accidentally broke his lance and banner on the archway while leaving Evesham Abbey. This was perceived as a bad omen for the forthcoming battle.
Battle
Simon de Montfort knew he and his army were trapped between Edward's army and the loop in the River Avon. Several hundred cavalrymen of Simon's army gathered at the foot of the hill, supported by the Welsh infantry, led by Humphrey (V) de Bohun, preparing to face Edward's army.
According to the chronicler William Rishanger, Simon, soaking wet in the downpour, looked at Edward's army bustling on top of the ridge called Green Hill, was reminded of his victory at the Battle of Lewes the previous year, and bitterly remarked:
"By the arms of St. James, they come on skillfully, but it is not by themselves, but from me that they have learned this. So let us commend our souls to God; for our bodies are theirs".
Edward's forces were to the east, with Gilbert de Clare commanding the west. Edward's combined forces were 10,000 strong, while de Montfort commanded only 5,000. Being outnumbered, de Montfort decided to concentrate his forces on the center of Edward's front, hoping to drive a wedge between Edward and de Clare.
Following his speech, Simon turned his horse and yelled, giving the signal to attack.
De Montfort's men, once again, wore white crosses on their right upper arms, as they did at Lewes. As the rain came down, their column of knights made their way up Green Hill, their proud banners glowing with the richness of the bearer's arms. It was then that Simon recognized the banner of Gilbert de Clare amongst those of Edward's forces and is said to have quipped, "That red dog will devour us today." As the rebels of de Montfort advanced, the King's army deployed from marching columns into line.
The Welsh infantry proved unreliable, lost faith, and deserted the battlefield early on, after Roger Mortimer's men crossed the bridge south of Evesham and attacked de Montfort's men from behind.

De Montfort's mounted cavalry were now on their own. His knights charged forward, colliding with their foes, columns of knights versus rows of royalist defenses. Initially, the knights drove deep into the lines, almost breaking a wedge between Edward's and de Clare's forces. Still, knights such as Warin de Bassingbourne steadied the shaken royalist forces, herding them back into formation.
The flanks of the royalist army swung inwards and surrounded de Montfort's forces, turning the battle into a massacre. With their defeat at Lewes still fresh in their minds, the royalists fought with bitterness, and no quarter was given. Despite attempts to surrender, most of the barons in de Montfort's force were killed in battle rather than taken prisoner.
De Montfort's son, Henry, taking station near his father's banner, is said to have led the charge but was cut down by a lance, knocked off his horse, and slain. When Simon de Montfort saw this, he told his men that it was time "for them to face their fate."
Despite the hopelessness of his position, Simon de Montfort would not flee nor surrender. His valour and steadfastness were all a knight should be, and he was worthy of respect. Then, Simon's horse was killed under him, and he continued the battle on foot. At least a dozen of Edward's knights surrounded de Montfort, but he would not surrender his sword. Finally Roger Mortimer swung the final and fatal blow.
Roger de Mortimer's body was mutilated with his head, hands, feet, and testicles cut off by William Maltravers. Hugh le Despenser also lay near de Montfort, killed in battle by the thrust of a dagger from Roger Mortimer. Other notable knights who had fallen in de Montfort's army were Peter de Montfort, Guy de Baliol, William de Mandeville, Ralph Basset of Drayton, Thomas of Astley, William of Birmingham, and Richard Trussel. Others, such as Henry de Hastings, David of Uffington, Nicholas de Segrave, and John de Vesci, were taken prisoner.
Simon de Montfort the Younger had led a large mounted army from Kenilworth to support his father, but they were greeted by the sight of men-at-arms fleeing the battle. Stunned and shamed by his tardiness in arriving, they turned and rode back to Kenilworth Castle.

Aftermath
King Henry III was present at the battle in De Montfort's custody, dressed in enemy armor and colors. Henry was rescued from the melee by Roger de Leybourne, a converted rebel. Roger had previously defended Rochester Castle against the barons before the Battle of Lewis. Henry's royal authoriity as King was restored.
The Battle of Evesham was over. By midday, the baronial army under de Montfort was destroyed, and any survivors had fled the field. Simon de Montfort's head was sent to Lady Mortimer at Wigmore as a gruesome memento.
King Henry III and Prince Edward permitted the monks to bury Simon de Montfort, his son Henry de Montfort, and Hugh le Despenser at the steps of the High Altar at Evesham Abbey.
In September 1265, at the Parliament at Winchester, all those who had taken part in the rebellion against the King were disinherited. Although Simon de Montfort was dead and the barons defeated, the rebellion was not over as pockets of resistance against the King remained, and the Second Barons' War would go on for another year and a half.