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.