By 1258 Simon was an active member of the baronial opposition that forced the king to turn over the power of government to a committee of 15 (of whom Simon was one), which ruled under the Provisions of Oxford, supplemented by the Provisions of Westminster of 1259. Divisions soon appeared in the baronial party, and in 1261, when a majority of the barons consented to an unfavorable compromise with the king, Simon left England. There was, however, renewed discontent in England following Henry's annulment (1262) of the provisions, and in 1263 Simon returned to assume leadership in the Barons' War.
Simon won a great victory at Lewes in 1264 and became master of England, which he intended to place under a form of government similar to that prescribed in the Provisions of Oxford. However, he could achieve no legal settlement with the king and so ruled as virtual military dictator. His famous Parliament of 1265, to which he summoned not only knights from each shire but also, for the first time, representatives from boroughs, was an attempt to rally national support, but at the same time he was alienating many of his baronial supporters. In 1265 his most powerful ally, Gilbert de Clare, 8th earl of Gloucester, deserted and with Prince Edward joined the nobles of the Welsh Marches to start the wars again. Simon de Montfort was defeated and killed at Evesham.
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