Simon de Montfort, 6th Earl of Leicester (c. 1205 – 4 August 1265), later sometimes referred to as Simon V de Montfort to distinguish him from his namesake relatives, was a nobleman of French origin and a member of the English peerage, who led the baronial opposition to the rule of King Henry III of England, culminating in the Second Barons' War. Following his initial victories over royal forces, he became de facto ruler of the country, and played a major role in the constitutional development of England.