Simon de Montfort, 6th Earl of Leicester (1208 – August 4, 1265) was the principal leader of the baronial opposition to king Henry III of England.

He was the youngest son of Simon de Montfort, a French nobleman, and Alix of Montmorency. His paternal grandmother was Amicia de Beaumont, the senior co-heiress to the Earldom of Leicester and a large estate in England, but King John of England did not allow anyone who already held property in France to take ownership of such an estate in England. As a young man, Simon probably took part in the Albigensian Crusades of the early 1220s.

When the elder Simon died in 1218, his elder son Amaury succeeded him. In 1229 the two brothers (Amaury and Simon) came to an arrangement whereby Simon gave up his rights in France and Amaury in turn gave up his rights in England. Thus freed from any allegiance to the king of France, Simon successfully petitioned for the English inheritance, which he received the next year, although he did not take full possession for several more years, and was not yet formally recognized as earl.

Meanwhile in January 1238 he secretly married Eleanor of England, daughter of King John of England, sister of King Henry III of England. Eleanor had previously been married to William Marshal, 2nd Earl of Pembroke , and she had sworn a vow of chastity on his death, which she broke by marrying Simon. The archbishop of Canterbury, Edmund Rich, condemned the marriage for this reason. The English nobles protested the marriage of the king's sister to a foreigner of modest rank; most notably, Eleanor's brother Richard rose up in revolt when he learned of the marriage. King Henry eventually bought off Richard with 6,000 marks and peace was restored.

Relations between King Henry and Simon were cordial at first. Henry lent him his support when Simon embarked for Rome in March 1238 to seek papal approval for his marriage. When Simon and Eleanor's first son was born in November 1238 (despite rumors, more than nine months after the wedding night), he was baptized Henry in honor of his royal uncle. In Febuary 1239 Simon was finally invested with the earldom of Leicester. He also acted as the king's counselor and was one of the godfathers of Henry's son, Edward.

Shortly after Prince Edward's birth, however, there was a falling out. Simon owed a great sum of money to Thomas II of Savoy, the uncle of Henry's queen, and named Henry as security for his repayment. King Henry had evidently not been told of this, and when he discovered that Simon had used his name, he was enraged. On August 9, 1239 Henry confronted Simon, called him an excommunicate and threatened to imprison him in the Tower of London. "You seduced my sister," King Henry said, "and when I discovered this, I gave her to you, against my will, to avoid scandal." Simon and Eleanor fled to France to escape the king's wrath. Having announced his intention to go on Crusade two years previously, Simon raised funds and finally set out for the Holy Land in summer 1240, leaving Eleanor in Brindisi, Italy. His force followed behind the much larger army led by his brother, Amaury. Also at the same time Simon's brother-in-law Richard took the cross, but their armies traveled separately. He arrived in Jerusalem by June 1241, when the citizens asked him to be their governor, but does not seem to have ever faced combat in the Holy Land. That autumn he left Syria and joined King Henry's campaign in Poitou. The campaign was a failure, and an exasperated Simon declared that Henry ought to be locked up like Charles the Simple.

Like his father, Simon was a hardened and ruthless soldier, as well as a capable administrator. His dispute with the king largely came about due to the latter's determination to ignore the swelling discontent within the country, caused by a combination of factors which included famine and a sense among the English barons that the king was too ready to dispense favour to his Poitevin and Savoyard relatives. In 1258, at Oxford, in his moment of greatest fame, Simon played a key role in calling a parliament which ranks as the forerunner of the modern institution. The king's son, the future King Edward I of England at first sympathised with Simon's cause, but later the two became enemies, and the Provisions of Oxford, which the king had sworn to uphold, were broken at the behest of the Pope in 1261.

Civil war broke out, and Simon de Montfort's army met and defeated the royal forces at the Battle of Lewes in 1264. The rebels captured Prince Edward, and the subsequent treaty set up a model parliament to agree a constitution formulated by Simon, the first parliament at which both knights and burgesses were present, thereby substantially broadening representation to include new groups of society.

However, many of the barons who had initially supported him now started to feel that Montfort's reforms were going too far, and his many enemies turned his triumph into disaster.

Matthew Paris reports that the bishop of Lincoln, Robert Grosseteste, once said to Simon's eldest son, "My beloved child, both you and your father will meet your deaths on one day, and by one kind of death, but it will be in the name of justice and truth." When Montfort faced the army of his nephew Edward, and saw the skill with which it was organized, he declared, "By St. James' arm, they are approaching with wisdom, and they learned this method from me, not from themselves. Let us then commend our souls to God, for our bodie are theirs."

Edward Longshanks' forces defeated and killed Montfort during the Barons' War at the Battle of Evesham in 1265, his body being mutilated, eviscerated and the remains scattered. His head was awarded to Roger Mortimer, who took it home as a gift for his wife. The precedent he set in calling his parliament with broader representation lived on, however, and was one that Edward would come to follow himself as king.

Montfort's family were forced into exile in his native France. His daughter, Eleanor, later married Llywelyn the Last as Simon had planned. After his death Simon's lands and titles became forfeit to the crown. A few months later the crown re-granted them to Edmund Crouchback, the king's youngest son.

Montfort has given his name to various English institutions, such as De Montfort University and De Montfort Hall, both in Leicester.

A memorial to Simon de Montfort stands in the park in Evesham in a place believed to be near where the High Altar of Evesham Abbey was located and a Stone Cross in the nearby Churchyard, the Stone Cross being viewable from the park. The memorial states that it is constructed with stone brought from near his birthplace in France.

Sources

• Maddicott, J.R. Simon de Montfort, 1996