Cornered, Montfort had to accept a truce and agree to submit the issue to arbitration by the French king Louis IX. By the Mise of Amiens, Louis decided entirely in favour of Henry, and repudiated the provisions. The settlement did not present a solution to the conflict, but rather a recipe for further problems. The one-sided decision for the king and against the barons left Montfort with little choice but armed rebellion. Hostilities started already in February, when Montfort's sons, Henry and Simon the Younger, attacked the possessions of Roger Mortimer in the Marches. Henry summoned the feudal army, and the royal forces won an important victory at Northampton, where the younger Simon was captured. Montfort was still in control of London, as Henry regained control over Kent and Sussex. Montfort marched out of London to negotiate, but the termsinvolving maintaining the provisionswere rejected by the king. The only option remaining was to fight, and the two forces met at Lewes on 14 May 1264. In spite of inferior numbers, the baronial forces led by Simon de Montfort won the battle. Edward, commanding the right wing, quickly defeated the London forces. When he set out in pursuit of the fleeing soldiers, however, he left the rest of the royal army exposed. The baronial forces took advantage of the situation, and soon won the day.