Hence it came about that, when war actually broke out in the spring of 1263, the elder of the Lords Marchers fought on the side of the king--such as Roger Mortimer and Humphrey de Bohun--though the younger men--young Gilbert of Gloucester and Humphrey de Bohun, the son of Hereford--remained under the spell of Simon de Montfort's fascination and high-minded enthusiasm. The war began in the Welsh Marches, Simon attacking the forces of Edward of Chester and Roger Mortimer--the principal royalists. As these were also the most formidable enemies of the Welsh, Llywelyn at the same time attacked them from the other side, the baronial party and Welsh co-operating, though without any formal alliance or friendly feelings. Thus in 1263 the baronial army besieged Shrewsbury, which defended itself till "a countless host" of Welshmen, came up and began to attack it from the other side; the town then surrendered to the barons lest it should fall into the hands of the Welsh.
This campaign led to a very great defection from the baronial side: the Lord Marchers generally--such as Clifford and Fitzalan--deserted Simon, who appeared as a traitor to the country. How great the defection is shown by Simon's words: "Though all should leave me, yet with my four sons I will stand true to the just cause, which I have sworn to uphold for the honour of the Church and the good of the kingdom; I have been in many lands, pagan and Christian, but in none have I found such faithlessness as in England."
The royalists were now the strongest party in the Marches, and in 1264 Edward and Mortimer gained a number of successes over the troops of Simon and Llywelyn (who seem to have been acting together) and captured Brecon. But they were called off to the main seat of war in the Midlands, and Simon inflicted a crushing defeat on the royalists at Lewes, in Sussex, 1264. It appears that Welsh archers fought in Simon's army, but these would be South Welsh, not North Welsh, the troops of Gilbert de Clare, not those of Llywelyn. The Marchers who escaped from Lewes were followed up by Simon, and being encircled by his forces and those of Llywelyn, submitted in December, 1264.
But Simon in the hour of triumph was now near his fall, which was made inevitable by the defection of Gilbert de Clare and whole of the Gloucester interest. The causes of the quarrel as given in the chronicles are mainly personal. Simon, with all his greatness, was quick-tempered and overbearing, inclined to seize power for himself, and perhaps even avaricious; one may infer this from the statement of a friendly chronicler, William Rishanger: "his habitual prayer to God was that he would save him from avarice and covetousness of worldly goods." But, apart from merely personal questions, it is to be noticed that the closer the relations between Simon and Llywelyn became, the less cordial became his relations to Gilbert de Clare. Thus when Simon co-operated with Llywelyn in bringing Mortimer and the Marchers to submission in December, 1264, Gilbert began to intrigue with them; and soon after the famous parliament of 1265 had transferred to Simon the earldom of Chester--thus relieving Llywelyn of his most dangerous neighbour, Prince Edward--Gilbert definitely joined Mortimer and Edward. The meeting between the three at Ludlow is very important; for Prince Edward now, at the instance of Gloucester, definitely pledged himself to the cause of reform and good government. It may be said for the Red Earl of Gloucester that in deserting Simon he did not desert his cause. To ensure the future of English liberties it was no longer necessary to support De Montfort: "henceforth it was not Simon but Edward who best represents the cause of orderly national progress."