In the summer of 1263, Edward and Montfort both raised armies. Leicester made himself master of Hereford, Gloucester, and Bristol, and when Edward threw himself into Windsor Castle, he occupied Isleworth, hoping to cut his enemy off from London, where the king and queen had taken refuge in the Tower. But the hostility of the Londoners made the Tower an uneasy refuge for them. On one occasion, when the queen attempted to make her way up the Thames in the hope of joining her son at Windsor, the citizens assailed her barge so fiercely from London Bridge that she was forced to return to the Tower. The foul insults which the rabble poured upon his mother deeply incensed Edward and he became a bitter foe of the city for the rest of his life. For the moment the hostility of London was decisive against Henry. Once more the king was forced to confirm the Provisions, agree to a fresh banishment of the aliens, and restore Hugh Despenser to the justiciarship. This was the last baronial triumph. In a few weeks Edward again took up arms, and was joined by many of Montfort's associates, including his cousin, Henry of Almaine. Even the Earl of Gloucester was wavering. The barons feared the appeal to arms, and entered into negotiations. Neither side was strong enough to obtain mastery over the other, and a recourse to arbitration seemed the best way out of an impossible situation. Accordingly, on December, 1263, the two parties agreed to submit the question of the validity of the Provisions to the judgment of Louis IX.
The king and his son at once crossed the channel to Amiens, where the French king was to hear both sides. A fall from his horse prevented Leicester attending the arbitration, and the barons were represented by Peter Montfort, lord of Beaudesert castle in Warwickshire, and representative of an ancient Anglo-Norman house that was not akin to the family of Earl Simon. Louis did not waste time, and on January 23, 1264, issued his decision in a document called the "Mise of Amiens," which pronounced the Provisions invalid, largely on the ground of the papal sentence. Henry was declared free to select his own wardens of castles and ministers, and Louis expressly annulled "the statute that the realm of England should henceforth be governed by native-born Englishmen". "We ordain," he added, "that the king shall have full power and free jurisdiction over his realm as in the days before the Provisions." The only consolation to the barons was that Louis declared that he did not intend to derogate from the ancient liberties of the realm, as established by charter or custom, and that he urged a general amnesty on both parties. In all essential points Louis decided in favour of Henry. Though the justest of kings, he was after all a king, and the limitation of the royal authority by a baronial committee seemed to him to be against the fundamental idea of monarchy. The pious son of the Church was biassed by the authority of two successive popes, and he was not unmoved by the indignation of his wife, the sister of Queen Eleanor. A few weeks later Urban IV. confirmed the award.
The Mise of Amiens was too one-sided to be accepted. The decision to refer matters to St. Louis had been made hastily, and many enemies of the king had taken no part in it. They, at least, were free to repudiate the judgment and they included the Londoners, the Cinque Ports, and nearly the whole of the lesser folk of England. The Londoners set the example of rebellion. They elected a constable and a marshal, and joining forces with Hugh Despenser, the baronial justiciar, who still held the Tower, marched out to Isleworth, where they burnt the manor of the King of the Romans. "And this," wrote the London Chronicler, "was the beginning of trouble and the origin of the deadly war by which so many thousand men perished." The Londoners did not act alone. Leicester refused to be bound by the award, though definitely pledged to obey it. It was, he maintained, as much perjury to abandon the Provisions as to be false to the promise to accept the Mise of Amiens. After a last attempt at negotiation at a parliament at Oxford, he withdrew with his followers and prepared for resistance. "Though all men quit me," he cried, "I will remain with my four sons and fight for the good cause which I have sworn to defend—the honour of Holy Church and the good of the realm." This was no mere boast. The more his associates fell away, the more the Montfort family took the lead. While Leicester organised resistance in the south, he sent his elder sons, Simon and Henry, to head the revolt in the midlands and the west.
There was already war in the march of Wales when Henry Montfort crossed the Severn and strove to make common cause with Llewelyn. But the Welsh prince held aloof from him, and Edward himself soon made his way to the march. At first all went well for young Montfort. Edward, unable to capture Gloucester and its bridge, was forced to beg for a truce. Before long he found himself strong enough to repudiate the armistice and take possession of Gloucester. Master of the chief passage over the lower Severn, Edward abandoned the western campaign and went with his marchers to join his father at Oxford, where he at once stirred up the king to activity. The masters of the university, who were strong partisans of Montfort, were chased away from the town. Then the royal army marched against Northampton, the headquarters of the younger Simon, who was resting there, and, on April 4, the king and his son burst upon the place. Their first assault was unsuccessful, but next day the walls were scaled, the town captured, and many leading barons, including young Simon, taken prisoner. The victors thereupon marched northwards, devastated Montfort's Leicestershire estates, and thence proceeded to Nottingham, which opened its gates in a panic.
Leicester himself had not been idle. While his sons were courting disaster in the west and midlands, he threw himself into London, where he was rapturously welcomed. The Londoners, however, became very unruly, committed all sorts of excesses against the wealthy royalists, and cruelly plundered and murdered the Jews. Montfort himself did not disdain to share in the spoils of the Jewry, though he soon turned to nobler work. He was anxious to open up communications with his allies in the Cinque Ports. But Earl Warenne, in Rochester castle, blocked the passage of the Dover road over the Medway. Accordingly Montfort marched with a large following of Londoners to Rochester, captured the town, and assaulted the castle with such energy that it was on the verge of surrendering. The news of Warenne's peril reached Henry in the midlands. In five days the royalists made their way from Nottingham to Rochester, a distance of over 160 miles. On their approach Montfort withdrew into London.
Flushed with their successes at Northampton and Rochester, the royalists marched through Kent and Sussex, plundering and devastating the lands of their enemies. Though masters of the open country, they had to encounter the resistance of the Clare castles, and the solid opposition of the Cinque Ports. Their presence on the south coast was specially necessary, for Queen Eleanor, who had gone abroad, was waiting, with an army of foreign mercenaries, on the Flemish coast, for an opportunity of sailing to her husband's succour. The royal army was hampered by want of provisions, and was only master of the ground on which it was camped. As a first fruit of the alliance with Llewelyn, Welsh soldiers lurked behind every hedge and hill, cut off stragglers, intercepted convoys, and necessitated perpetual watchfulness. At last the weary and hungry troops found secure quarters in Lewes, the centre of the estates of Earl Warenne.
Montfort then marched southwards from the capital. Besides the baronial retinues, a swarm of Londoners, eager for the fray, though unaccustomed to military restraints, accompanied him. On May 13 he encamped at Fletching, a village hidden among the dense oak woods of the Weald, some nine miles north of Lewes. A last effort of diplomacy was attempted by Bishop Cantilupe of Worcester who, despite papal censures, still accompanied the baronial forces. But the royalists would not listen to the mediation of so pronounced a partisan. Nothing therefore was left but the appeal to the sword.
The royal army was the more numerous, and included the greater names. Of the heroes of the struggle of 1258 the majority was in the king's camp, including most of the lords of the Welsh march, and the hardly less fierce barons of the north, whose grandfathers had wrested the Great Charter from John. The returned Poitevins with their followers mustered strongly, and the confidence of the royalists was so great that they neglected all military preparations. The poverty of Montfort's host in historic families attested the complete disintegration of the party since 1263. Its strength lay in the young enthusiasts, who were still dominated by the strong personality and generous ideals of Leicester, such as the Earl of Gloucester, or Humphrey Bohun of Brecon, whose father, the Earl of Hereford, was fighting upon the king's side. Early on the morning of May 14 Montfort arrayed his troops and marched southward in the direction of Lewes. Dawn had hardly broken when the troops were massed on the summit of the South Downs, overlooking Lewes from the north-west.
Lewes is situated on the right bank of a great curve of the river Ouse, which almost encircles the town. To the south are the low-lying marshes through which the river meanders towards the sea, while to the north, east, and west are the bare slopes of the South Downs, through which the river forces its way past the gap in which the town is situated. To the north of the town lies the strong castle of the Warennes, wherein Edward had taken up his quarters, while in the southern suburb the Cluniac priory of St. Pancras, the chief foundation of the Warennes, afforded lodgings for King Henry and the King of the Romans. When Simon reached the summit of the downs, his movements were visible from the walls. But the royal army was still sleeping and its sentinels kept such bad watch that the earl was able to array his troops at his leisure.
From the summit of the hills two great spurs, separated by a waterless valley, slope down towards the north and west sides of the town. The more northerly led straight to the castle, and the more southerly to the priory. Montfort's plan was to throw his main strength on the attack on the priory, while deluding the enemy into the belief that his chief object was to attack the castle. He was not yet fully recovered from his fall from his horse, and it was known that he generally travelled in a closed car or horse-litter. This vehicle he posted in a conspicuous place on the northerly spur, and planted over it his standard. In front of it were massed the London militia, mainly infantry and the least effective element in his host. Meanwhile the knights and men-at-arms were mustered on the southerly spur under the personal direction of Montfort, who held himself in the rear with the reserve, while the foremost files were commanded by the young Earl of Gloucester, whom Simon solemnly dubbed to knighthood before the assembled squadrons. Then the two divisions of the army advanced towards Lewes, hoping to find their enemies still in their beds.