From A Great and Terrible King: Edward I and the Forging of Britain, by Marc Morris (Pegasus, 2015), Kindle p. 60:
Siege and counter-siege, raid and counter-raid: such was the normal method of medieval warfare. Skilled commanders moved their troops like pieces on a chessboard, taking individual castles and knights as part of a developing strategy. Attrition and retaliation were the name of the game; direct confrontation was to be avoided at all costs. No matter how daring a general might be, he would almost never commit to battle because of the enormous risk involved. In the noise and confusion of a battle everything could be lost in a few short hours. As a consequence, they were rare events: in the spring of 1264, there had been no battle in England for almost half a century.
Montfort, a renowned warrior well into his mid-fifties, had never fought in one. And yet it was battle that Montfort now sought. In recent weeks his range of options had diminished rapidly. After his retreat to London they had never seemed so limited or so bleak. Dover Castle, his only other significant asset, was now threatened by the arrival in the south of the royal army; once it fell, Montfort would be trapped. In strategic terms it was almost checkmate, but the earl was not a man readily to concede defeat. On 6 May, like a cornered animal, he came out fighting, marching his forces out of London in search of his enemies.