With an army reinforced by 15,000 Londoners, he marched to the relief of the Cinque Ports, which were now threatened by the King. Even on the march he was forsaken by many of the nobles who followed him. Halting at Fletching in Sussex, a few miles from Lewes, where the royal army was encamped, Earl Simon with the young Earl of Gloucester offered the King compensation for all damage if he would observe the Provisions. Henry's answer was one of defiance, and though numbers were against him the Earl resolved on battle. His skill as a soldier reversed the advantages of the ground; marching at dawn he seized the heights eastward of the town, and moved down these slopes to an attack. His men, with white crosses on back and breast, knelt in prayer before the battle opened. Edward was the first to open the fight; his furious charge broke the Londoners on Leicester's left, and in the bitterness of his hatred he pursued them for four miles, slaughtering three thousand men. He returned to find the battle lost.

Crowded in the narrow space with a river in their rear, the royalist centre and left were crushed by Earl Simon; the Earl of Cornwall, now King of the Romans, who, as the mocking song of the victors ran, "makede him a castel of a mulne post" ("he weened that the mill-sails were mangonels "goes on the sarcastic verse), was made prisoner, and Henry himself captured. Edward cut his way into the Priory only to join in his father's surrender.

The victory of Lewes placed Earl Simon at the head of the state. "Now England breathes in the hope of liberty," sang a poet of the time; "the English were despised like dogs, but now they have lifted up their head and their foes are vanquished." The song announces with almost legal precision the theory of the patriots. " He who would be in truth a king, he is a 'free king' indeed if he rightly rule himself and his realm. All things are lawful to him for the government of his kingdom, but nothing for its destruction. It is one thing to rule according to a king's duty, another to destroy a kingdom by resisting the law." "Let the community of the realm advise, and let it be known what the generality, to whom their own laws are best known, think on the matter. They who are ruled by the laws know those laws best, they who make daily trial of them are best acquainted with them; and since it is their own affairs which are at stake, they will take more care, and will act with an eye to their own peace." "It concerns the community to see what sort of men ought justly to be chosen for the weal of the realm." The constitutional restrictions on the royal authority, the right of the whole nation to deliberate and decide on its own affairs, and to have a voice in the selection of the administrators of government, had never been so clearly stated before.

But the moderation of the terms agreed upon in the Mise of Lewes, a convention between the King and his captors, shows Simon's sense of the difficulties of his position. The question of the Provisions was again to be submitted to arbitration; and a parliament in June, to which four knights were summoned from every county, placed the administration till this arbitration was complete in the hands of a new council of nine, to be nominated by the Earls of Leicester and Gloucester and the patriotic Bishop of Chichester. Responsibility to the community was provided for by the declaration of a right in the body of barons and prelates to remove either of the Three Electors, who in turn could displace or appoint the members of the Council. Such a constitution was of a different order from the cumbrous and oligarchical Committees of 1258. But the plans for arbitration broke down, Lewis refused to review his decision, and the Pope formally condemned the barons' cause. The Earl's difficulties thickened every day. The Queen gathered an army in France for an invasion, and the barons on the Welsh border were still in arms. It was impossible to make binding terms with an imprisoned King, yet to release Henry without terms was to renew the war.

A new parliament was summoned in January, 1265, to Westminster, but the weakness of the patriotic party among the baronage was shown in the fact that only twenty-three earls and barons could be found to sit beside the hundred and twenty ecclesiastics. But it was just this sense of his weakness that drove Earl Simon to a constitutional change of mighty issue in our history. As before, he summoned two knights from every county. But he created a new force in English politics when he summoned to sit beside them two citizens from every borough. The attendance of delegates from the towns had long been usual in the county courts when any matter respecting their interests was in question; but it was the writ issued by Earl Simon that first summoned the merchant and the trader to sit beside the knight of the shire, the baron, and the bishop in the parliament of the realm.

It is only this great event however which enables us to understand the large and prescient nature of Earl Simon's designs. Hardly a few months had passed since the victory of Lewes, and already, when the burghers took their seats at Westminster, his government was tottering to its fall. Dangers from without the Earl had met with complete success; a general muster of the national forces on Barham Down put an end to the projects of invasion entertained by the mercenaries whom the Queen had collected in Flanders; the threats of France died away into negotiations; the Papal Legate was forbidden to cross the Channel, and his bulls of excommunication were flung into the sea. But the difficulties at home grew more formidable every day. The restraint upon Henry and Edward jarred against the national feeling of loyalty, and estranged the mass of Englishmen who always side with the weak. Small as the patriotic party among the barons had always been, it grew smaller as dissensions broke out over the spoils of victory. The Earl's justice and resolve to secure the public peace told heavily against him.