On this day of 14 May in 1264 King Henry III marched out of Lewes to confront the army of his brother-in-law Simon de Montfort, whose troops had assembled on the heights overlooking the town. Henry had every reason to be confident of victory that day. The war between him and Simon had gone completely in his favour since he stormed Northampton a little over a month earlier. It left Simon bottled up in London, helpless to prevent the king from dishing out a punishing foray into his earldom of Leicester. He managed to draw Henry south by attempting to take Rochester, but that only allowed the royalists to tighten the ring around London. The most the rebels could do was station their archers in the woods to pick off the king’s men. When Henry’s cook was killed by one of these snipers, he had a group of insurgents summarily beheaded, the first time he executed prisoners of war in 40 years. He was preparing the fleet of the Channel ports to launch an amphibious assault on London when word reached him that Simon had left, desperate to gamble everything on one pitched battle.
As the action got underway, Simon’s left was completely crushed by a cavalry charge under Henry’s son Edward. Simon had a reserve force ready in case they wheeled around and drove into his exposed flank, but to his great fortune Edward kept up his pursuit until it took him out of the battle. That was Simon’s chance. He threw his reserves against the royalist centre, which was commanded by Henry’s brother Richard of Cornwall, and shattered their line. Richard and his men ran for it, leaving Simon to focus his attack on Henry. The king, who was fighting at the front, managed to retreat to the walled priory outside the town and set up a defensive perimeter there. Simon was unable to take it or the castle, and Edward could well turn the tide by returning with reinforcements from nearby garrisons. But he compounded his earlier mistake by choosing instead to fight his way into the priory. Simon’s victory was now certain, but in order to secure it he needed both the king and heir to come out alive. For that, he was forced to make concessions that led to his demise the following year.