The English statesman and soldier Simon de Montfort, 6th Earl of Leicester (1208-1265), led the opposition to Henry III and played a major role in constitutional development.

Simon de Montfort, born in Normandy, was the fourth and youngest son of Simon de Montfort IV and Alice de Montmorency. After spending his first years in Normandy, where in 1210 his father had been deprived of his English estates, Simon decided in 1229 to go to England with his elder and sole brother, Almeric, to claim the earldom of Leicester by right of his English grandmother. He soon became a royal favorite, made friends with such important people as Robert Grosseteste, and on Aug. 13, 1231, was given livery of the estates. In 1236 Simon was seneschal at the coronation of the Queen, and 2 years later he married Eleanor, the sister of Henry III and widow of William Marshall, Earl of Pembroke.

Simon was formally...