John Giffard left him because he refused to allow him to exact ransom from a prisoner contrary to the agreement made after Lewes. The young Earl Gilbert of Gloucester, though enriched with the estates of the foreigners, resented Leicester's prohibition of a tournament, his naming the wardens of the royal castles by his own authority, and his holding Edward's fortresses on the Welsh marches by his own garrisons. Gloucester's later conduct proves the wisdom of Leicester's precautions. In the spring Parliament of 1265 he openly charged the Earl with violating the Mise of Lewes, with tyranny, and with aiming at the crown. Before its close he withdrew to his own lands in the west, and secretly allied himself with Roger Mortimer and the Marcher barons. Earl Simon soon followed him to the west, taking with him the King and Edward. He moved along the Severn, securing its towns, advanced westward to Hereford, and was marching at the end of June along bad roads into the heart of South Wales to attack the fortresses of Earl Gilbert in Glamorgan when Edward suddenly made his escape from Hereford and joined Gloucester at Ludlow. The moment had been skilfully chosen, and Edward showed a rare ability in the movements by which he took advantage of the Earl's position.
Moving rapidly along the Severn he seized Gloucester and the bridges across the river, destroyed the ships by which Leicester strove to escape across the Channel to Bristol, and cut him off altogether from England. By this movement too he placed himself between the Earl and his son Simon, who was advancing from the east to his father's relief. Turning rapidly on this second force Edward surprised it at Kenilworth and drove it with heavy loss within the walls of the castle. But the success was more than compensated by the opportunity which his absence gave to the Earl 0f breaking the line of the Severn. Taken by surprise and isolated as he was, Simon had been forced to seek for aid and troops in an avowed alliance with Llewelyn, and it was with Welsh reinforcements that he turned to the east. But the seizure of his ships and of the bridges of the Severn held him a prisoner in Edward's grasp, and a fierce attack drove him back, with broken and starving forces, into the Welsh hills. In utter despair he struck northward to Hereford; but the absence of Edward now enabled him on the 2nd of August to throw his troops in boats across the Severn below Worcester. The news drew Edward quickly back in a fruitless counter-march to the river, for the Earl had already reached Evesham by a long night march on the morning of the 4th, while his son, relieved in turn by Edward's counter-march, had pushed in the same night to the little town of Alcester. The two armies were now but some ten miles apart, and their junction seemed secured.
But both were spent with long marching, and while the Earl, listening reluctantly to the request of the King, who accompanied him, halted at Evesham for mass and dinner, the army of the younger Simon halted for the same purpose at Alcester.
"Those two dinners doleful were, alas!" sings Robert of Gloucester; for through the same memorable night Edward was hurrying back from the Severn by country cross-lanes to seize the fatal gap that lay between them. As morning broke his army lay across the road that led northward from Evesham to Alcester. Evesham lies in a loop of the river Avon where it bends to the south; and a height on which Edward ranged his troops closed the one outlet from it save across the river. But a force had been thrown over the river under Mortimer to seize the bridges, and all retreat was thus finally cut off. The approach of Edward's army called Simon to the front, and for the moment he took it for his son's. Though the hope soon died away a touch of soldierly pride moved him as he recognized in the orderly advance of his enemies a proof of his own training. "By the arm of St. James," he cried, "they come on in wise fashion, but it was from me that they learnt it" A glance however satisfied him of the hopelessness of a struggle; it was impossible for a handful of horsemen with a mob of half-armed Welshmen to resist the disciplined knighthood of the royal army. " Let us commend our souls to God," Simon said to the little group around him, "for our bodies are the foe's." He bade Hugh Despenser and the rest of his comrades fly from the field. " If he died," was the noble answer, " they had no will to live." In three hours the butchery was over.
The Welsh fled at the first onset like sheep, and were cut ruthlessly down in the cornfields and gardens where they sought refuge. The little group of knights around Simon fought desperately, falling one by one till the Earl was left alone. So terrible were his sword-strokes that he had all but gained the hill-top when a lance-thrust brought his horse to the ground, but Simon still rejected the summons to yield, till a blow from behind felled him, mortally wounded, to the ground. Then with a last cry of" It is God's grace" the soul of the great patriot passed away.