Henry III was an unpopular monarch due to his autocratic style, displays of favouritism and his refusal to negotiate with his barons. The barons eventually imposed a constitutional reform known as the Provisions of Oxford upon Henry that called for a thrice-yearly meeting led by Simon de Montfort to discuss matters of government. Henry sought to escape the restrictions of the provisions and applied to Louis IX of France to arbitrate in the dispute. Louis agreed with Henry and annulled the provisions. Montfort was angered by this and rebelled against the King along with other barons in the Second Barons' War.
The war was not initially openly fought, each side toured the country to raise support for their army. By May the King's force had reached Lewes where they intended to halt for a while to allow reinforcements to reach them. The King encamped at St. Pancras Priory with a force of infantry, but his son, Prince Edward (later King Edward I), commanded the cavalry at Lewes Castle 500 yards (460 m) to the north. De Montfort approached the King with the intention of negotiating a truce or failing that to draw him into open battle. The King rejected the negotiations and de Montfort moved his men from Fletching to Offam Hill, a mile to the north-west of Lewes, in a night march that surprised the royalist forces.
The royalist army was up to twice the size of de Montfort's. Henry held command of the centre, with Prince Edward, William de Valence, 1st Earl of Pembroke, and John de Warenne, 6th Earl of Surrey, on the right; and Richard, 1st Earl of Cornwall, and his son, Henry of Almain, on the left. The barons held the higher ground, overlooking Lewes and had ordered their men to wear white crosses as a distinguishing emblem. De Montfort split his forces into four parts, giving his son, Henry de Montfort command of one quarter; Gilbert de Clare with John FitzJohn and William of Montchensy another; a third portion consisting of Londoners was placed under Nicholas de Segrave whilst de Montfort himself led the fourth quarter with Thomas of Pelveston.
The baronial forces commenced the battle with a surprise dawn attack on foragers sent out from the royalist forces. The King then made his move. Edward led a cavalry charge against Seagrave's Londoners, placed on the left of the baronial line, that caused them to break and run to the village of Offham. Edward pursued his foe for some four miles, leaving the King unsupported. Henry was forced to launch an attack with his centre and right divisions straight up Offham Hill into the baronial line which awaited them at the defensive. Cornwall's division faltered almost immediately but Henry's men fought on until compelled to retreat by the arrival of de Montfort's men that had been held as the baronial reserve.
The King's men were forced down the hill and into Lewes where they engaged in a fighting retreat to the castle and priory. Edward returned with his weary cavalrymen and launched a counterattack but upon locating his father was persuaded that, with the town ablaze and many of the King's supporters having fled, it was time to accept de Montfort's renewed offer of negotiations. The Earl of Cornwall was captured by the barons when he was unable to reach the safety of the priory and, being discovered in a windmill, was taunted with cries of "Come down, come down, thou wicked miller."