With the Battle of Lewes, Montfort had won control of royal government, but after the defection of several close allies and the escape from captivity of Prince Edward, he found himself on the defensive. Forced to engage the royalists at Evesham, he faced an army twice the size of his own. The battle soon turned into a massacre; Montfort himself was killed and his body mutilated. Though the battle effectively restored royal authority, scattered resistance remained until the Dictum of Kenilworth was signed in 1267.
Simon de Montfort, 6th Earl of Leicester, had gained a dominant position in the government of the Kingdom of England after his victory at the Battle of Lewes a year earlier. He also held the King, Prince Edward, and the King's brother Richard of Cornwall in his custody. However, his sphere of influence began to shrink rapidly, owing to loss of key allies. In February, Robert de Ferrers, Earl of Derby was arrested and imprisoned in the Tower. An even more important collaborator, Gilbert de Clare, the Earl of Gloucester, deserted to the side of the King in May of the same year. With Gloucester's assistance, Prince Edward escaped from Montfort's captivity.
With the Lords of the Welsh Marches now in rebellion, Montfort solicited the aid of Llywelyn ap Gruffudd, the Prince of Wales. Llywelyn agreed to help, in return for full recognition of his title and the promise that he could keep all military gains. Whatever benefits this alliance might have brought Montfort, the large concessions cost him popularity at home. Meanwhile, Edward laid siege to the town of Gloucester, which fell on 29 June. Montfort's goal now became to unite with the forces of his son Simon, and engage with the royal army, but the younger Simon moved much too slowly westwards from London. Eventually Simon made it to the baronial stronghold of Kenilworth, but Edward managed to inflict great losses on Simon's forces, many of whom were quartered outside the castle walls. From there the Prince moved south, where on 4 August, using many banners captured at Kenilworth to deceive Montfort into thinking his reinforcements were arriving, he managed to trap the older Montfort in a loop of the Avon, blocking off the only bridge and thereby forcing Montfort to fight without his son's reinforcements. When Montfort realized this, he allegedly commented: "May the Lord have mercy upon our souls, as our bodies are theirs."
Heeding a lesson learned at the Battle of Lewes, the royals took position on the high ground. Along a ridge called Green Hill, just north of Evesham, Edward set up his forces on the left, with Gloucester commanding the right. At about eight in the morning, Montfort left the town of Evesham as a great thunderstorm began to rage. At Lewes, the baronial forces had gained confidence to win the day by a sense of divine destiny, reinforced by white crosses on their uniforms. This time the royal army had taken their lead, and wore a red cross as their distinguishing mark. According to the chronicler William Rishanger, when Montfort saw the advance of the royal troops, he exclaimed that "They have not learned that for themselves, but were taught it by me."
The respective forces of the baronial and royal armies have been estimated to be 5,000 and 10,000 strong. Montfort, facing such unfavourable numbers, decided to concentrate his forces on the centre of the enemy’s front, hoping to drive a wedge through the line. Though the tactics were initially successful, the baronial forces soon lost the initiative, especially as the Welsh infantry provided by Llywelyn ap Gruffudd had proved unreliable, and deserted at an early point. The flanks of the royal army closed in on Montfort's, surrounding them. With Montfort confronted by a force twice the size of his own, on unfavourable ground, the battle rapidly turned into a massacre.